Implicitly set prefix lengths for Doltgres

[fulghum]

This change allows Doltgres customers to use TEXT columns in secondary indexes and UNIQUE constraints.

MySQL requires that users specify a prefix length when using a TEXT or BLOB column in a secondary index (also in the primary key, but Dolt/GMS doesn't support that currently). PostgreSQL does not have a concept of prefix lengths. Including the full TEXT content in an index introduces problems with our 65kb row size limit and allowing a secondary index to include an address and de-ref it in TupleComparator is a larger/messier change. Implicitly setting a prefix length is a pragmatic way to get the right behavior, without doing the large refactoring to allow secondary indexes to support address-encoded values correctly. That work seems to be required before we can support TEXT columns being used in primary keys, too.

I played with a few ways to structure this behavior difference. My first choice way to refactor out a separate analyzer rule that Doltgres could insert, but that turned into a lot of copy and paste that didn't seem easier to maintain, so I tried adding a EngineType property to Analyzer so that code can be aware of what type of engine it is emulating and switch on that for different behaviors. I may still take another pass at a separate analyzer rule, just to try that approach again – putting that analyzer rule in GMS, instead of in Doltgres, could prevent some of the copy/paste, without having to export additional, small helper functions.

[fulghum]

Closing this and switching to a different approach that requires less changes in GMS