Adds an API to clone an index. This is similar to the index split and shrink APIs, just with the
difference that the number of primary shards is kept the same. In case where the filesystem
provides hard-linking capabilities, this is a very cheap operation.
Indexing cloning can be done by running `POST my_source_index/_clone/my_target_index` and it
supports the same options as the split and shrink APIs.
Closes#44128
Dan Hermann
Lisa Cawley
James Rodewig
Yannick Welsch