Improve downstream primary key decision for sink

[xx01cyx]

Considering the fact that the downstream is accessible to the schema and might be able to generate a pk (e.g. JDBC), we could perform a not-so-strict primary key check on the sink:

1. Try to fetch downstream's pk info.
2. If the downstream has its own pk, use the downstream's pk. If not, use user-defined pk. If the user hasn't defined a pk, then we should reject the sink DDL.

Note that this should be done in meta, and should be part of sink validation.

Related: #7045 (comment)