Protect get-snapshots APIs against excessive memory use

[DaveCTurner]

The get-snapshots APIs accumulate the details of all matching snapshots in memory before responding. In a high-index-count cluster with a long retention period on snapshots this can add up to GiB+ of heap on the elected master to request all snapshots.

We should make this API more efficient for sure, see #95345, but we should also protect the cluster against accidental use of this API to request too many snapshots by rejecting such requests before they can cause harm.

[elasticsearchmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[DaveCTurner]

In fact once #104851 is done we could also make this API run in near-constant memory, streaming its results in chunks all the way from the master node to the client.