Today you can convert a searchable snapshot index back into a regular index by
restoring the underlying snapshot, but this is somewhat wasteful if the shards
are already in cache since it copies the whole index from the repository again.

Instead, we can make use of the locally-cached data by using the clone API to
copy the contents of the cache into the layout expected by a regular shard.
This commit marks the searchable snapshot's private index settings as
`NotCopyableOnResize` so that they are removed by resize operations such as
cloning.

Cloning a regular index typically hard-links the underlying files rather than
copying them, but this is tricky to support in the case of a searchable
snapshot so this commit takes the simpler approach of always copying the
underlying files.

Today if we try to shrink or to split a searchable snapshot 
index using the Resize API a new index will be created 
but can't be assigned, and even if it was assigned it won't 
work as the number of shards can't be changed and must 
always match the number of shards from the snapshot.

This commit adds some verification to prevent a snapshot 
backed indices to be resized and if an attempt is made, 
throw a better error message.

Note that cloning is supported since #56595 and in this 
change we make sure that it is only used to convert the 
searchable snapshot index back to a regular index.

Relates #74977 (comment)

Today if we try to shrink or to split a searchable snapshot
index using the Resize API a new index will be created
but can't be assigned, and even if it was assigned it won't
work as the number of shards can't be changed and must
always match the number of shards from the snapshot.

This commit adds some verification to prevent a snapshot
backed indices to be resized and if an attempt is made,
throw a better error message.

Note that cloning is supported since elastic#56595 and in this
change we make sure that it is only used to convert the
searchable snapshot index back to a regular index.

Relates elastic#74977 (comment)

)

Today if we try to shrink or to split a searchable snapshot
index using the Resize API a new index will be created
but can't be assigned, and even if it was assigned it won't
work as the number of shards can't be changed and must
always match the number of shards from the snapshot.

This commit adds some verification to prevent a snapshot
backed indices to be resized and if an attempt is made,
throw a better error message.

Note that cloning is supported since #56595 and in this
change we make sure that it is only used to convert the
searchable snapshot index back to a regular index.

Relates #74977 (comment)