backupccl: add feature flag support for BACKUP, RESTORE

[jseldess]

PR: cockroachdb/cockroach#56533

From release notes:

Added cluster settings to enable/ disable the BACKUP and RESTORE commands. Attempts to use these features while they are disabled returns an error indicating that the database administrator has disabled the feature. Example usage: SET CLUSTER SETTING feature.backup.enabled = FALSE; SET CLUSTER SETTING feature.backup.enabled = TRUE; SET CLUSTER SETTING feature.restore.enabled = FALSE; SET CLUSTER SETTING feature.restore.enabled = TRUE;. [#56533][#56533] {% comment %}doc{% endcomment %}

[mwang1026]

Hmm.... I think SQL did this project?

[lnhsingh]

I'm going to kick this over to the SQL writers for now

[ericharmeling]

@mwang1026

I think SQL did this project?

I believe that it is.

@vy-ton

From https://cockroachlabs.atlassian.net/browse/CRDB-2219:

We don’t plan on publicly documenting these cluster settings but we should document them internally for CC SREs.

Closing this for now.