Register backends on the fly

[gordonwatts]

This code was designed to enable a local version of SX to run, in process, to help wtih debugging and development.

• Adds extra object to the Endpoint to track a special adaptor.
• Ability to register a new end point on the fly, which then gets appended to the list of read end points.
• ServiceXClient properly creates a specialized client when requested., and one for the minio for_transform.
• Add ServiceXAdaptor and MinioAdaptor as a protocal.

Notes:

• pydantic can't seem to deal with duck typing Protocals, so object is getting used.
• It might be ok to remove the protocol totally - especially if the pydantic problem can't be solved. Or make them sub-classes of a ABC.