Add Take function in the migration package.

[blmhemu]

Currently, the pool.go defines Take as the primary way to take a connection.
Ideally if the migrations package can follow the same interface, it would be easy for interoperability.

But migrations package does not provide Take, instead it provides Get(ctx) (conn, error) (which is different from the Get(ctx) conn in pool.go)

Changing the signature of Get would be a breaking change, but adding Take would instead atleast make a interoperable interface.

[zombiezen]

SGTM. Thanks!