planner: support hypo-indexes

[qw4990]

Feature Request

Is your feature request related to a problem? Please describe:

Support hypo-indexes, which can help the index advisor to know whether some indexes are worthwhile for some particular queries.

Describe the feature you'd like:

1. Support to use create index {name} type hypo on {table} (c1, c2) to create hypo-indexes;
2. Support to use drop index to remove hypo-indexes;
3. Show create table can show all hypo-indexes;

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

PG's hypo index: https://hypopg.readthedocs.io/en/rel1_stable/