CDRIVER-2386 refactor retryable writes into shared function (#1576)

Adds a new private function `mongoc_cluster_run_retryable_write` and removes duplicate retry logic. No functional changes are expected.

* rename `is_retryable` to `can_retry`

Intended to better describe. `is_retryable_write` describes if the command is eligible for retry (regardless of whether retry occurred). `can_retry` is initially true if `is_retryable_write` is true. `can_retry` is set to false after a retry (can only retry once).

* simplify condition

`_mongoc_write_error_update_if_unsupported_storage_engine` only applies if the server does not support retryable writes. Apply regardless of whether a retry occurred.

* remove `_mongoc_client_retryable_write_command_with_stream`

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Co-authored-by: Ezra Chung <88335979+eramongodb@users.noreply.github.com>