retry config transactions on Unavailable errors

[alexandernorth]

Problem

A data-server "not connected" error (device unreachable) was classified as
non-recoverable, so the Config was marked FailedUnRecoverable and never
retried resulting in a config applied while the device was down being stuck in failed state,
even after the device recovered.

Change

• Classify recoverability by gRPC code via isRecoverableTransactionError
• targetconfig reconciler: use exponential backoff, bounded to 1s–60s, replacing the fixed 500ms retry

Pairs-with: sdcio/data-server#466