feature: add support for additional inputs

[mfridman]

goose currently supports -dir, which reads migration files either from disk or from an embedded filesystem.

We should consider adding support for multiple inputs, such as:

• remote git repository (GitHub, Gitlab, etc.)
• remote buckets (S3, GCS, etc)

1. Design a generic interface and implement the existing inputs. The interface must be extensible to allow additional implementations beyond the ones listed above.
2. Weave it through the library
3. Add concrete implementations for the remotes mentioned above