feat: Add build information

[PythonGermany]

Summary

Add application wide build informations via

• startup via info log
• UI footer with link to github release page (configurable if it should be displayed)
  • TODO: Don't add link to keep it simple? -> Removed
  • TODO?: Only add link to github releases page for correct version pattern ^v\d+\.\d+\.\d+$
  • TODO?: Make link to release page configurable. If it is enabled and/or where it points to.
• prometheus endpoint metric gatus_build_info
  • Add go_version label

Closes #833
Closes #901

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
  • Test workflows
    • Version
    • Build date
    • Commit hash/revision
  • Test docker and direct build
    • Check version with and without tag
      • via make
      • via docker
    • Check commit hash via make, docker and directly building (without setting any custom -X ldflags)
    • Check build date via make and docker and directly building (without setting any custom -X ldflags)
  • Test new ui.show-version option (make sure if it is false that the version is not embedded in the UI files) -> Version placeholder in index.html is empty if set to false
• Updated documentation in README.md, if applicable.

[PythonGermany]

Currently implemented ways to get the build info and info types available:

• Logs: Version, Revision, Revision date
• UI: Version (configurable)
• Prometheus metrics endpoint: Version, Revision, Revision date, Go version

[PythonGermany]

@TwiN Would exposing the build info via the API be of use and a wanted addition? Or would that be redundant/irrelevant/unwanted since the API already has it's own version?

[TwiN]

Exposing it via the UI is enough for now.

At some point, I imagine we'll also have to expose it via cli (gatus version or gatus --version), but there's no need to implement this right now.