Merge #74685

74685: rfcs: introduce rfc to support protected timestamps in multi-tenancy r=irfansharif a=arulajmani

_Link to the [RFC text](https://github.com/arulajmani/cockroach/blob/multi-tenant-pts-rfc/docs/RFCS/20211111_multitenant_protected_timestamps.md)._

The protected timestamp subsystem (PTS) provides a mechanism for long
running operations such as backup and CDC to prevent data they are
operating on from getting garbage collected.

The PTS subsystem was designed before CRDB supported multi-tenancy,
and as such, only applies to host tenant ranges. This RFC proposes to
leverage the span configuration infrastructure as the transport
mechanism for protected timestamps. This allows secondary tenants
to lay protected timestamps as well, making the long running operations
mentioned above more robust.

The RFC only proposes changing the transport mechanism for the PTS
subsystem; the mechanism by which protection is guaranteed in KV
remains the same. However, the RFC does propose expressing protection
in terms of schema objects as opposed to static spans to unlock new
backup functionality for the future.

Release note: None

Co-authored-by: arulajmani <[email protected]>