Minor updates to support UPDATE ... FROM in Doltgres

[fulghum]

Minor changes to support UPDATE ... FROM in Doltgres (dolthub/doltgresql#1536)

While working on this, I noticed a few issues with the UPDATE ... JOIN support in GMS:

• There was duplicated logic for UPDATE ... JOIN in PlanBuilder.buildUpdate() and in the analyzer rule modifyUpdateExprsForJoin. Not all UPDATE ... JOIN statements can be handled in PlanBuilder, because some statements, like IN SUBQUERY, are transformed into a join during analysis, so we need logic in the analyzer to identify UPDATE ... JOIN plans after that transformation. To reduce the duplication, I removed the logic from PlanBuilder and now we rely on on the modifyUpdateExprsForJoin rule to mark all UPDATE ... JOIN plans.
• Callers use plan.GetUpdatable() to get a reference to the table being updated, but UPDATE ... JOIN can update multiple tables. The GetUpdatable() APIs should be refactored to model this, otherwise we cannot accurately track multiple tables being modified and apply all the correct triggers/constraints.
• Currently UPDATE ... JOIN never honors foreign key constraints, due to limitations in the concrete type UpdateJoin.GetUpdatable() returns. The updatableJoinTable type does not implement sql.ForeignKeyTable, so GMS is unable to load any foreign key constraints from it. This also needs to be handled in a future PR.

[jycor]

LGTM