Add "origin" string (or metadata) to CreateSnapshotRequest

[dakrone]

In #38461 we are working towards a snapshot lifecycle management where snapshots can automatically be taken. One thing that would be helpful, however, is to know how a snapshot originated (like which policy created the snapshot).

It would be great if we could extend the CreateSnapshotRequest (and optionally the REST layer) to allow providing an "origin" (or metadata, if we want to be more generic) string when creating the snapshot. This would then show up when the snapshot was listed from the repository.

/cc @original-brownbear

[elasticmachine]

Pinging @elastic/es-distributed

[ywelsch]

I would prefer to have a general-purpose _meta field (like we have for mappings) rather than an "origin" field.

[gwbrown]

This has been implemented in #41281.