[feature/internal/httpclient] add option to configure outgoing protocol (#4134)

# Description

Adds a configuration option to set the outgoing transport of HTTP requests for testing purposes.

## Checklist

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [ ] 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.
- [ ] 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.
- [ ] I/we have run tests and they pass locally with the changes.
- [ ] I/we have run `go fmt ./...` and `golangci-lint run`.
  - Note: `go fmt` yes, golangci-lint fails, can't load any configuration

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4134
Reviewed-by: kim <gruf@noreply.codeberg.org>
Co-authored-by: famfo <famfo@famfo.xyz>
Co-committed-by: famfo <famfo@famfo.xyz>

test/envparsing.sh

@@ -105,6 +105,7 @@ EXPECT=$(cat << "EOF"
     "host": "example.com",
     "http-client-allow-ips": [],
     "http-client-block-ips": [],
+    "http-client-insecure-outgoing": false,
     "http-client-timeout": 30000000000,
     "http-client-tls-insecure-skip-verify": false,
     "instance-allow-backdating-statuses": true,