[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/config/config.go

@@ -55,6 +55,7 @@ func fieldtag(field, tag string) string {
 // https://github.com/mvdan/gofumpt.
 type Configuration struct {
 	LogLevel           string `name:"log-level" usage:"Log level to run at: [trace, debug, info, warn, fatal]"`
+	LogFormat          string `name:"log-format" usage:"Log output format: [logfmt, json]"`
 	LogTimestampFormat string `name:"log-timestamp-format" usage:"Format to use for the log timestamp, as supported by Go's time.Layout"`
 	LogDbQueries       bool   `name:"log-db-queries" usage:"Log database queries verbosely when log-level is trace or debug"`
 	LogClientIP        bool   `name:"log-client-ip" usage:"Include the client IP in logs"`