feat(experimental): add collect migrations logic

[mfridman]

This PR adds the Source type that represents SQL and Gp migrations files found in the file system.

Source files are an intermediate representation that gets merged with registered Go migrations to return a Migration object. (this is an internal implementation detail, most users don't need to know this).

This implementation keeps parity with the existing code, with 3 notable exceptions:

1. Exit early if there are Go migrations in the file system that have not been registered at the time of initializing the Provider. Previously, we used to gather all the migrations but failed ONLY if/when running the migration, which leads to partial errors at runtime. Note, that the existing implementation remains as-is, only the Provider code is different.

Specifically, this check:

goose/migration.go

Lines 103 to 105 in fe8fe97

	if !m.Registered {
	return fmt.Errorf("ERROR %v: failed to run Go migration: Go functions must be registered and built into a custom binary (see https://github.com/pressly/goose/tree/master/examples/go-migrations)", m.Source)
	}

2. This implementation allows us to expose a goose.WithGo and goose.WithGoNoTx options to register migrations directly into the provider instead of relying on the init() registration functions.

This is particularly useful for DI codebases that create a closure over the Go migration functions to pass down configs, other clients, etc.

3. Offer the ability to exclude migrations by filename using goose.WithExcludes as an option. Feature request: baseline/ignore migrations #431

---

One thought I had as I was implementing this is it becomes the caller's responsibility to know how to work with fs.FS.

I do think it's way more flexible for the caller, e.g., using embed.FS or fs.Sub, and it becomes less for goose to maintain, i.e., having an explicit dirpath and a fs.FS.