Rename purely interface related iteration of endorsement to attestation

As stated in https://research-development.nomadic-labs.com/nairobi-announcement.html:

With this protocol proposal, we begin this process by launching the deprecation of the endorsing_rights RPC endpoint, introducing attestation_rights as a replacement. This symbolic step is not breaking, as both old and new names would be available if Nairobi is activated on Mainnet. In future protocol proposals, we intend to go beyond symbolic steps though, and may have to introduce breaking changes in API names – which will be communicated with ample notice.

So we should not change the code nor the RPCs related to this newly introduced renaming until after the next protocol O.

Partially fix #253 (closed) (should not be closed until after the change above is done).

!354 (merged) should be merged before this MR.