sql: add AstDisplay impl for PgColumn

[JLDLaughlin]

Follow up to #6207.

This change adds checks to fail if Postgres CDC sources are created on top of tables that have COLLATE or DEFAULT column constraints.

This change is 

[petrosagg]

Hm, I don't that's quite right because we do support upstream tables that have default values and collate modifiers in their columns.

What we don't support is specifying those options in our CREATE SOURCE statement. When we're replicating from upstream we just get the data of the rows, we don't really care if something was created because a user inserted or it was a default value or if a string went through collation logic. In the end we get the raw data as it was finally stored upstream.

For example, it's perfectly fine for upstream to have a table created like this:

CREATE TABLE foo (a int DEFAULT 42);

But when we create the source on materialize the user should only be able to specify the types:

CREATE SOURCE p FROM POSTGRES <bla bla> TABLE foo (a int); -- allowed
CREATE SOURCE p FROM POSTGRES <bla bla> TABLE foo (a int DEFAULT 42); -- invalid

This is because those modifiers affect rows as they are being written, but we're only reading them out

[JLDLaughlin]

Thanks @petrosagg for reviewing, that totally makes sense! Updated this branch to just have the AstDisplay change.

[philip-stoev]

@JLDLaughlin many apologies for missing that review! I will make another pass at testing postgresql sources once all the pieces arrive in "main" (including the actual implementation).