spannertest: add support for dropping Foreign Keys

[gmontagu]

Is your feature request related to a problem? Please describe.
When testing a SQL migration DDL statement and attempting to drop a Foreign Key from a table, spannertest throws the error: (details: rpc error: code = Unimplemented desc = unhandled DDL table alteration type spansql.DropConstraint)

Describe the solution you'd like
Support for DROP CONSTRAINT call when using spannertest.

Describe alternatives you've considered
Forgoing adding the DDL statement to the migration script and manually running the command against spanner. But then we lose the ability to spin up new in memory spanner instances with this constraint droppd.

Additional context
Additional options are also not yet supported:
SET OPTIONS - ALTER TABLE {table} ALTER COLUMN {column} SET OPTIONS (allow_commit_timestamp=true);