[performance] use our own typed value context types for Value() key checking to improve performance (#4316)

Replaces our gtscontext package context.Context handling with our own typed contexts instead of `context.WithValue()`.

I wrote a quick benchmark consisting of (printlns to stop the compiler optimizing instructions away):
```golang
func BenchmarkContexts(b *testing.B) {
	var receiving *gtsmodel.Account
	var requesting *gtsmodel.Account
	var otherIRIs []*url.URL

	b.RunParallel(func(pb *testing.PB) {
		for pb.Next() {
			ctx := context.Background()

			ctx = gtscontext.SetBarebones(ctx)
			ctx = gtscontext.SetFastFail(ctx)
			ctx = gtscontext.SetDryRun(ctx)
			ctx = gtscontext.SetReceivingAccount(ctx, receiving)
			ctx = gtscontext.SetRequestingAccount(ctx, requesting)
			ctx = gtscontext.SetOtherIRIs(ctx, otherIRIs)

			if !gtscontext.Barebones(ctx) {
				println("oh no!")
			}

			if !gtscontext.IsFastfail(ctx) {
				println("oh no!")
			}

			if !gtscontext.DryRun(ctx) {
				println("oh no!")
			}

			if gtscontext.ReceivingAccount(ctx) != nil {
				println("oh no!")
			}

			if gtscontext.RequestingAccount(ctx) != nil {
				println("oh no!")
			}

			if len(gtscontext.OtherIRIs(ctx)) > 0 {
				println("oh no!")
			}
		}
	})
}
```

Before results:
```shell
kim @ ~/Projects/main/gts.4
--> go test -v -run=none -bench=.* -benchmem ./internal/gtscontext/ -count=5
goos: linux
goarch: amd64
pkg: code.superseriousbusiness.org/gotosocial/internal/gtscontext
cpu: AMD Ryzen 7 7840U w/ Radeon  780M Graphics
BenchmarkContexts
BenchmarkContexts-16            19050348                61.73 ns/op          288 B/op          6 allocs/op
BenchmarkContexts-16            18245772                61.71 ns/op          288 B/op          6 allocs/op
BenchmarkContexts-16            18853680                61.80 ns/op          288 B/op          6 allocs/op
BenchmarkContexts-16            18561621                62.67 ns/op          288 B/op          6 allocs/op
BenchmarkContexts-16            17819241                62.89 ns/op          288 B/op          6 allocs/op
PASS
ok      code.superseriousbusiness.org/gotosocial/internal/gtscontext    6.112s
```

After results:
```shell
kim @ ~/Projects/main/gts.4
--> go test -v -run=none -bench=.* -benchmem ./internal/gtscontext/ -count=5
goos: linux
goarch: amd64
pkg: code.superseriousbusiness.org/gotosocial/internal/gtscontext
cpu: AMD Ryzen 7 7840U w/ Radeon  780M Graphics
BenchmarkContexts
BenchmarkContexts-16            28038618                41.67 ns/op          144 B/op          6 allocs/op
BenchmarkContexts-16            26537552                42.50 ns/op          144 B/op          6 allocs/op
BenchmarkContexts-16            26720542                42.39 ns/op          144 B/op          6 allocs/op
BenchmarkContexts-16            27408031                43.15 ns/op          144 B/op          6 allocs/op
BenchmarkContexts-16            25597026                44.02 ns/op          144 B/op          6 allocs/op
PASS
ok      code.superseriousbusiness.org/gotosocial/internal/gtscontext    5.997s
```

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4316
Co-authored-by: kim <grufwub@gmail.com>
Co-committed-by: kim <grufwub@gmail.com>

1 files changed, 53 insertions, 0 deletions

diff --git a/internal/gtscontext/context_test.go b/internal/gtscontext/context_test.go
new file mode 100644
index 000000000..cf61acb4a
--- /dev/null
+++ b/internal/gtscontext/context_test.go
@@ -0,0 +1,53 @@
+package gtscontext_test
+
+import (
+	"context"
+	"net/url"
+	"testing"
+
+	"code.superseriousbusiness.org/gotosocial/internal/gtscontext"
+	"code.superseriousbusiness.org/gotosocial/internal/gtsmodel"
+)
+
+func BenchmarkContexts(b *testing.B) {
+	var receiving *gtsmodel.Account
+	var requesting *gtsmodel.Account
+	var otherIRIs []*url.URL
+
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			ctx := context.Background()
+
+			ctx = gtscontext.SetBarebones(ctx)
+			ctx = gtscontext.SetFastFail(ctx)
+			ctx = gtscontext.SetDryRun(ctx)
+			ctx = gtscontext.SetReceivingAccount(ctx, receiving)
+			ctx = gtscontext.SetRequestingAccount(ctx, requesting)
+			ctx = gtscontext.SetOtherIRIs(ctx, otherIRIs)
+
+			if !gtscontext.Barebones(ctx) {
+				println("oh no!")
+			}
+
+			if !gtscontext.IsFastfail(ctx) {
+				println("oh no!")
+			}
+
+			if !gtscontext.DryRun(ctx) {
+				println("oh no!")
+			}
+
+			if gtscontext.ReceivingAccount(ctx) != nil {
+				println("oh no!")
+			}
+
+			if gtscontext.RequestingAccount(ctx) != nil {
+				println("oh no!")
+			}
+
+			if len(gtscontext.OtherIRIs(ctx)) > 0 {
+				println("oh no!")
+			}
+		}
+	})
+}