[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>

0 files changed, 0 insertions, 0 deletions