- Rename `Get()` to `GetURL()`.
- Add `GetBlob()` to retrieve the media file blobs in
filesystem and and s3 media providers.
This enables reading media files inside listmonk paving the way to
campaign file attachments.
- Fix path related issues in filesystem and S3.
- Add checks for S3 "/" path prefix.
- Add support for custom S3 domain names.
- Remove obsolete `width` and `height` columns from media table (breaking)
- Add `provider` field to media table (breaking)
