v2.3: validator: Add --wait-for-exit flag to exit subcommand (backport of #6780)

[mergify]

Problem

Copied from #6233

Currently, agave-validator exit will return immediately when the AdminRpc function call returns, even though the actual validator process might still be running. This isn't ideal if someone is trying to do something like:

agave-validator exit && ./restart_validator_script.sh

Summary of Changes

• Add a new pid method to AdminRpc interface
• Add a new --wait-for-exit flag option to exit subcommand
  • It works by querying the PID from a running validator, initiating exit (same as before) and then using waiting until the returned PID is no longer alive

Previous Attempt

This change was previously introduced in #6233; however, there was a minor issue discoverd that affects upgrade scenarios. Namely, if a "new" agave-validator binary (one that calls pid()) is used to exit an "old" agave-validator (one that predates this change), it would error out since the old bin wouldn't recognize the pid() request.

This is mitigated by calling pid() BUT only checking the result if using --wait-for-exit. Since the new flag is opt-in, it seems more reasonable to expect that someone who opted in to a new feature would keep an eye on it. Once a version with this change is widely adopted, pid() will be available and more direct error handling will work just fine

Since this is re-adding a reverted commit, I left the original two commits as-is and added a third commit for the change in behavior. Hopefully this makes things easier for folks who might have previously reviewed this change

---

This is an automatic backport of pull request #6780 done by Mergify.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 35.21127% with 46 lines in your changes missing coverage. Please review.

Project coverage is 82.8%. Comparing base (9963221) to head (3971515).

Additional details and impacted files

@@            Coverage Diff            @@
##             v2.3    #6908     +/-   ##
=========================================
- Coverage    82.8%    82.8%   -0.1%     
=========================================
  Files         850      850             
  Lines      379824   379883     +59     
=========================================
+ Hits       314710   314754     +44     
- Misses      65114    65129     +15     

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