Draft: [Pg] Add support for procedural functions

[luca-nardelli]

Hi all, this PR is my attempt at adding support for postgres functions to Drizzle.

Syntax:

pgFunction('uppercase', {
	schema: 'public',
	args: 'input_text text',
	language: 'sql',
	returns: 'text',
	stability: 'stable',
	security: 'invoker',
	body: 'SELECT UPPER(input_text)',
})

While I did add some tests (basically only in drizzle-kit), I'm not 100% confident to have covered everything since this repo is quite big, so I'd ask for some review by the maintainers before proceeding further (that's why I'm marking this as draft). Right now it's possible to define functions, introspect functions and generate migrations from the schema snapshot. Every time a function changes, it will be dropped and recreated (to keep things simpler instead of ALTER-ing it).

Right now, not all function parameters are supported, but they should be easily added if the proposed approach works.

This PR also forces singlestore to use linux/amd64 image since it's not available on arm and tests were failing then.

Related issues/discussions:
#2586
#2434