Add a way to specify to only search the SOARClient with a Fido search

[hayesla]

Describe the feature

At the moment, when you query for data using Fido it returns all available options (e.g. if the data is available from the VSO and the SOAR, a Fido search would return two results from each).

There are use cases when you may only want to search the SOAR, so registering the a.Provider attr woul dbe good so that you could do

Fido.search(a.Time("2022-03-30 00:00", "2022-03-30 01:00"), a.Instrument("EUI"), a.Provider("SOAR"))

Proposed solution

i'll add this as a PR now