fix(spansql): Support UUID as a base data type

[adamhicks]

Currently if we have this ddl statement

CREATE TABLE tname (id UUID) PRIMARY KEY (id)

and we run it through spansql like this:

out, _ := spansql.ParseDDL("", ddl)
ddl = out.List[0].SQL()

we end up with

CREATE TABLE tname (
  id `UUID`,
) PRIMARY KEY(id)

which can no longer be run against the database as the UUID data type has been mangled.

This change proposes to add UUID as a base type so that it can be recognised and retained through parsing.

[gemini-code-assist]

Code Review

This pull request adds support for the UUID data type in spansql, fixing an issue where the type was being mangled during parsing and serialization. The changes correctly introduce UUID as a base type, update the parser and SQL generator, and add corresponding tests. The existing comment in spanner/spannertest/funcs.go has been kept as it provides valid feedback for code clarity and consistency.