[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>

example/config.yaml

@@ -36,6 +36,19 @@ log-db-queries: false
 # Default: true
 log-client-ip: true
 
+# String. Format to use for formatting log entries.
+# Supports "logfmt" and "json", with examples below:
+#
+# logfmt:
+# func=router.(*Router).Start.func1 level=INFO msg="listening on 127.0.0.1:8080"
+#
+# json:
+# {"func":"router.(*Router).Start.func1", "level":"INFO", "msg":"listening on 127.0.0.1:8080"}
+#
+# Examples: ["logfmt", "json"]
+# Default: "logfmt"
+log-format: "logfmt"
+
 # String. Format to use for the timestamp in log lines.
 # If set to the empty string, the timestamp will be
 # ommitted from the logs entirely.