Update Agent to support backpressure from server

[achawla2012]

Proposed changes

• Command server and metrics server to send gRPC Unavailable status error code if server is not accepting request for certain duration.
• Server to send Pause command to Agent (Added Proto) having information about backoffsettings.
• Agent would then update its backoff settings according to Backoff-settings sent by server in pause command.
• Refactored BackoffSettings struct.
  • Moved BackoffSettings to package backoff
  • Update to

    type BackoffSettings struct {
        InitialInterval time.Duration
        MaxInterval     time.Duration
        MaxElapsedTime  time.Duration
        Multiplier      float64
        Jitter          float64
    }
    

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

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