This commit splits the current ClusterBlockLevel.METADATA into two disctins ClusterBlockLevel.METADATA_READ and ClusterBlockLevel.METADATA_WRITE blocks. It allows to make a distinction between
an operation that modifies the index or cluster metadata and an operation that does not change any metadata.
Before this commit, many operations where blocked when the cluster was read-only: Cluster Stats, Get Mappings, Get Snapshot, Get Index Settings, etc. Now those operations are allowed even when
the cluster or the index is read-only.
Related to #8102, #2833Closes#3703Closes#5855Closes#10521Closes#10522
tlrx