Add command line flag to disable confirmation prompt for setting retention lock

If you run `gsutil retention lock gs://my-bucket` you will get a confirmation prompt like:
```
This will PERMANENTLY set the Retention Policy on gs://my-bucket to:

Retention Policy (UNLOCKED):
Duration: 3 Day(s)
Effective Time: Tue, 22 Mar 2022 16:53:19 GMT

This setting cannot be reverted! Continue? [y|N]:
```
This requires interactive input. I can't do it in an automated script. Please add a `--force` flag that bypasses the prompt.

Start at the command-line entry point for `gsutil retention lock` and trace how its confirmation prompt is handled. Add a `--force` option that bypasses the prompt while preserving the default confirmation behavior, then verify the command works in automated use.

Written by the indexing model from the issue text.