backupccl: call AllocateIDs after adding index mutations

[stevendanna]

While we expect that all of the index mutations added here to have
fully populated IDs and columns. Changes to the AddIndexMutation API
will soon require that AllocateIDs is called after any add index
mutation.

Release note: None

[cockroach-teamcity]

This change is 

[stevendanna]

Pulled this out of a larger branch to highlight it for review in case there is some reason I'm not seeing that we weren't calling AllocateIDs here.

[adityamaru]

I'm not super paged in to all the changes happening in AddIndexMutation but this diff seems fine on its own. I read through AllocateIDs and its comment seems to suggest it'll only allocate IDs for column, family, or index which has an ID of 0. Which seems like the right thing to regardless?

[stevendanna]

I agree that this seems like the right thing to do regardless and it should be safe.

bors r=adityamaru

[craig]

Build succeeded:

• GitHub CI (Cockroach)