Allow varchar columns as primary keys

[zachmu]

The problem: the varchar type in pgtypes was declared as Text, not Varchar. This triggered validation checks in both GMS and Dolt when such a column was used in a primary key. I originally tried adding additional index / length info in the CREATE TABLE transformation, but with the varchar type being interpreted as Text, there's no combination of params that can satisfy all the validation checks.

The error before this fix is e.g.

"ERROR: blob/text column 't' used in key specification without a key length"

Given that we have a lot of validation logic with key prefix lengths related to BLOB / TEXT types, it seems like this type really can't be declared as TEXT, but open to other ideas.

Collation / charset support is still fake and will need to be fixed, but no worse than it was before.

[Hydrocharged]

Looks fine, but I'd prefer if we just removed the validation roadblock in GMS, rather than making changes here. Also seems far simpler to disable the GMS roadblock, but can't confirm that. We also can't have TEXT as a primary key, which that would fix (since this PR only addresses VARCHAR). Could be more types too, not sure.

[zachmu]

Looks fine, but I'd prefer if we just removed the validation roadblock in GMS, rather than making changes here. Also seems far simpler to disable the GMS roadblock, but can't confirm that. We also can't have TEXT as a primary key, which that would fix (since this PR only addresses VARCHAR). Could be more types too, not sure.

Yes, we'll need to loosen the key type restrictions or make them customizable to support text as a primary key. Hard to do without breaking a bunch of stuff in GMS and Dolt. Bigger project.