fix(firestore): guard against gRPC call was cancelled exceptions

[dlarocque]

gRPC calls (ClientCall) can be cancelled due to network changes, timeouts, or channel resets. After a call is cancelled, all subsequent operations throw IllegalStateException: call was cancelled exception. In the SDK, there are paths where operations run on a already cancelled call. When these exceptions are unhandled, they go up to AsyncQueue and trigger AsyncQueue.panic(), crashing the application.

This was first reported in #6883, where call.halfClose operation is ran on a call that was already cancelled, causing crashes. This issue was closed by #6894, which wrapped the operation in a try/catch, and logged the error instead. This did not fully fix the issue, since 1) there are other other operations that can run on a cancelled call, which were not wrapped in try/catch (e.g. writeRequest -> call.sendMessage()), 2) operations can run in the AsyncQueue executor, and cannot be easily wrapped in a try/catch. These cases were reported recently in #8601.

This change addresses the two additional cases; the call.sendMessage() call is guarded, and a guardedGrpcExecutor was created to catch the call was cancelled exceptions from gRPC channels.

Notes:

• I wasn't able to reproduce the reported crashes locally.
• The guardedGrpcExecutor only catches IllegalStateException: call was cancelled with message containing call was cancelled. Any other exceptions here that are causing crashes, will still cause crashes.

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