Fixed support for newer Gencode GTF versions

[jamesemery]

Fixes #7166 #7385

This doesn't contain any of the necessary work to support Gencode GFF3 files yet #, that will (probably) come in a subsequent PR as it requires a much more substantial refactoring effort of the Gencode datasources code.

[droazen]

@jamesemery How "future-proof" is this PR? That is, how likely is it that future releases of Gencode will break the parser again? Has the parser been relaxed to the point where it will tolerate the addition of new fields, etc.?

[jamesemery]

@droazen I tried to relax absolutely everything but i stopped short of that. I have a test that any of the optional fields we previously special cased can now have any arbitrary value in them (which was the problem that sunk us here). However if they make DRASTIC changes to future gencode releases (like adding new top level transcript types or inventing a new reference orientation than + or -) then all bets are off...

[jonn-smith]

Looks pretty good. A few questions and change requests.

[jamesemery]

@jonn-smith responded to comments and back to you. I fully got rid of the old unparsed string of "anonymousOptionalFields" that were occasionally relevant for non-blessed fields in gencode. There is an enum list of the known optional fields for gencode but everything gets parsed into the same key-value list which should make phase 2 easier to finish here.

[jonn-smith]

Looks good!