feat: add pg-schema as a new diff tool

[sweatybridge]

What kind of change does this PR introduce?

PoC stripe/pg-schema-diff#85

What is the new behavior?

$ supabase db diff --schema public --use-pg-schema
Diffing schemas: public
Creating shadow database...
Setting up initial schema...
Creating custom roles supabase/roles.sql...
Finished supabase db diff on branch pg-diff.

-- HAS_UNTRACKABLE_DEPENDENCIES: This sequence has no owner, so it cannot be tracked. It may be in use by a table or function.
CREATE SEQUENCE "public"."employees_id_seq"
	AS integer
	INCREMENT BY 1
	MINVALUE 1 MAXVALUE 2147483647
	START WITH 1 CACHE 1 NO CYCLE
;
CREATE TABLE "employees" (
	"id" integer NOT NULL,
	"name" text COLLATE "pg_catalog"."default",
	"age" smallint NOT NULL
);
CREATE UNIQUE INDEX CONCURRENTLY employees_pkey ON public.employees USING btree (id);
ALTER TABLE "public"."employees" ADD CONSTRAINT "employees_pkey" PRIMARY KEY USING INDEX "employees_pkey";

TODO:

• find out why shadow database is created twice
• support remote -> local diff (currently impl requires a db factory)
• support more pg entities, including custom types, RLS policies, etc

Additional context

1. create an empty database: supabase db start
2. create a schema file and save to supabase/migrations/0_schema.sql

create table public.employees (
    id integer primary key generated always as identity,
    name text,
    age smallint not null
);

[coveralls]

Pull Request Test Coverage Report for Build 7720255566

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• -106 of 109 (2.75%) changed or added relevant lines in 3 files are covered.
• 9 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.6%) to 58.085%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cmd/db.go	1	6	16.67%
internal/db/diff/schema.go	0	101	0.0%

Files with Coverage Reduction	New Missed Lines	%
cmd/db.go	1	59.52%
internal/debug/postgres.go	3	64.86%
internal/gen/keys/keys.go	5	11.76%

Totals
Change from base Build 7692683417:	-0.6%
Covered Lines:	6157
Relevant Lines:	10600

---

💛 - Coveralls

[sweatybridge]

renamed from migra.go

[sweatybridge]

renamed from diff.go