Dataprovider - get canonical / MANE transcript

[davmlaw]

There is often the need for picking 1 transcript for a gene. This is often referred to as the canonical transcript, and nowadays (clinically) is usually the MANE transcript

It would be good to add a method to data provider to be able to retrieve the MANE transcript from a gene name

Transcripts could also have a field/flag on them saying whether they are MANE transcripts

Once we have the data provider API done, implementations like UTA or cdot could implement them

This is necessary to implement #517

Also came up as a request for #743