feat: Expose setter for global Go registry

[mfridman]

This PR introduces 2 functions to interact with the global go migration registry. Note, not safe for concurrent use!

Important

Once #379 fully lands, users are encouraged to register Go migrations directly on the goose.Provider, and not globally within the package scope.

Bonus, unlike the init() registration, this setter allows mixing up and down migrations with different transaction levels for the same version. For example, a caller can register an Up function that runs with *sql.Tx (transaction) and a Down function that runs with *sql.DB (no transaction).

Example

// Register one or more migrations globally
err = goose.SetGlobalMigrations(
	goose.Migration{
		Registered:        true,
		Version:           2,
		Source:            "00002_foo.sql",
		Type:              goose.TypeGo,
		UpFnContext:       func(context.Context, *sql.Tx) error { return nil },
		DownFnNoTxContext: func(context.Context, *sql.DB) error { return nil },
	},
)

// Reset global registry
goose.ResetGlobalMigrations()

Supersedes #351