Add a self-upgrader into the standalone binary

[aler9]

Describe the feature

Discussed in #3307

Requirements:

• This must not be available in Docker or when the server is provided by a package manager
• There must be no automatic calls to public endpoints, only manual
• Two new flags: mediamtx --check-updates and mediamtx --upgrade

[github-actions]

This issue is mentioned in release v1.15.2 🚀
Check out the entire changelog by clicking here