Implement debug service and its CLI commands

[gabrielcorado]

Implements RFD 167:

• Introduces a new service called "debug" which has endpoints for collecting pprof profiles and changing the log level. This service uses a Unix domain socket listener (instead of TCP).
• Adds teleport debug sub-commands to consume the service endpoints from the same machine.

changelog: New commands for debugging/troubleshooting Teleport instances. teleport debug set-log-level enables changing the instance log level without a restart. Also, the teleport debug profile can collect pprof profiles.

Note: Docs updates will be done after the implementation is merged.

[gabrielcorado]

I moved the debug HTTP mux, handlers, and constants to a separate package. It required a change in our .gitignore as directories named debug were ignored.

[public-teleport-github-review-bot[bot]]

@gabrielcorado See the table below for backport results.

Branch	Result
branch/v15	Failed