[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>

testrig/config.go

@@ -57,7 +57,8 @@ func InitTestConfig() {
 func testDefaults() config.Configuration {
 	return config.Configuration{
 		LogLevel:                   envStr("GTS_LOG_LEVEL", "error"),
-		LogTimestampFormat:         "02/01/2006 15:04:05.000",
+		LogFormat:                  envStr("GTS_LOG_FORMAT", "logfmt"),
+		LogTimestampFormat:         envStr("GTS_LOG_TIMESTAMP_FORMAT", "02/01/2006 15:04:05.000"),
 		LogDbQueries:               true,
 		ApplicationName:            "gotosocial",
 		LandingPageUser:            "",