Add Idleness Http Endpoint

[4e6]

Pull Request Description

close #1829

PR implements an GET / _idle request that returns a number of seconds the Language Server is idle (since the last request from IDE). The endpoint is required to implement auto shutdown feature in the cloud.

Important Notes

Checklist

Please include the following checklist in your PR:

• The documentation has been updated if necessary.
• All code conforms to the Scala, Java, and Rust style guides.
• All documentation and configuration conforms to the markdown and YAML style guides.
• All code has been tested where possible.

[iamrecursion]

This is missing documentation in the LS API docs.

[iamrecursion]

Great, comprehensive docs!

[iamrecursion]

Only thing is that you forgot to add it to the README.md in the language-server directory.