[YSQL] Support multiple colocation groups in one database

[m-iancu]

Currently for colocated tables, we have one colocation group per database (associated with the database).
In some cases though, several mostly-independent groups of tables (e.g. in different schemas) can exist in the same database and benefit from colocation within the group but adding all to the same group limits scalability (since they are all on the same tablet).
In such cases having more than one colocation group in a database (e.g. one per schema) can help find the best tradeoff between latency and throughput/scalability for the use-case.