[feature + performance] add JSON logging format (#4355)

# Description

Adds JSON logging as an optional alternative log output format. In the process this moves our log formatting itself into a separate subpkg to make it more easily modular, and improves caller name getting with some calling function name caching.

## Checklist

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [x] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [x] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4355
Co-authored-by: kim <grufwub@gmail.com>
Co-committed-by: kim <grufwub@gmail.com>

internal/log/pool.go

@@ -23,22 +23,22 @@ import (
 	"codeberg.org/gruf/go-byteutil"
 )
 
-// bufPool provides a memory pool of log buffers.
-var bufPool = sync.Pool{
-	New: func() any {
-		return &byteutil.Buffer{
-			B: make([]byte, 0, 512),
-		}
-	},
-}
+// bufPool provides memory
+// pool of log buffers.
+var bufPool sync.Pool
 
 // getBuf acquires a buffer from memory pool.
 func getBuf() *byteutil.Buffer {
 	buf, _ := bufPool.Get().(*byteutil.Buffer)
+	if buf == nil {
+		buf = new(byteutil.Buffer)
+		buf.B = make([]byte, 0, 512)
+	}
 	return buf
 }
 
-// putBuf places (after resetting) buffer back in memory pool, dropping if capacity too large.
+// putBuf places (after resetting) buffer back in
+// memory pool, dropping if capacity too large.
 func putBuf(buf *byteutil.Buffer) {
 	if buf.Cap() > int(^uint16(0)) {
 		return // drop large buffer