mb should support setting default CMEK key

## Desired Behavior
I would like to be able to create a bucket w/ default CMEK key in one line like this:
```bash
gsutil mb gs://${BUCKET_NAME} -p ${PROJECT} -k ${KEY_RESOURCE}
```
This has enough information to attempt the three steps under the hood w/o the user running separate commands as described in the [public GCS CMEK docs](https://cloud.google.com/storage/docs/encryption/using-customer-managed-keys#gsutil_2)

## Current Behavior
Currently to create a bucket and set a CMEK key one must
1. create a bucket w/ `gsutil mb gs://${BUCKET_NAME}`
1. authorizes the GCS project SA w/ `gsutil kms authorize ${PROJECT} -k ${KEY_RESOURCE}`
1. sets default key with `gsutil kms encryption -k ${KEY_RESOURCE} gs://${BUCKET_NAME}`

Trace the gsutil mb command and compare its flow with kms authorize and kms encryption, using the linked public GCS CMEK documentation as the behavioral reference. Done means mb accepts the proposed key option and performs bucket creation, authorization, and default-key setup in one invocation.

Written by the indexing model from the issue text.