When altering rf for a keyspace, all tablets in this ks will get more replicas. Part of this process is rebuilding tablets' onto new node(s). This PR extends the tablets transition code to support rebuilding of tablet on new replica.
fixes: #18030Closesscylladb/scylladb#18082
* github.com:scylladb/scylladb:
test: Check data presense as well
test: Test how tablets are copied between nodes
test: Add sanity test for tablet migration
api: Add method to add replica to a tablet
tablet: Make leaving replica optional
0c74c2c12f