Do not allow put mapping on follower

[dnhatn]

Today, the mapping on the follower is managed and replicated from its
leader index by the ShardFollowTask. Thus, we should prevent users
from modifying the mapping on the follower indices.

Relates #30086

[martijnvg]

Left one comment, but LGTM. When I thought about this, I also saw no other way to implement this without introducing an extension point.

[jasontedor]

The change looks good but I have one comment about the naming. We use "origin" as a concept in the security code base and I am concerned about the "origin" here being confused with the "origin" there. Since we are handing the entire mapping request to the validator (and not really any notion of "origin" or "source") I would be inclined to change the name to remove "origin" and simply say that they are put mapping request validators. What do you think?

[dnhatn]

We use "origin" as a concept in the security code base and I am concerned about the "origin" here being confused with the "origin" there. Since we are handing the entire mapping request to the validator (and not really any notion of "origin" or "source") I would be inclined to change the name to remove "origin" and simply say that they are put mapping request validators.

+1. I pushed d2a61d5 to remove the origin from the validator and documentation. @jasontedor Could you please have another look? Thank you!

[jasontedor]

LGTM.

[dnhatn]

Thanks @martijnvg and @jasontedor for reviewing.