[ENSCORESW-3106] BCF parser
Created by: tgrego
This PR brings a BCF parser to ensembl-io.
BCF is a compiled version of VCF, thus a writer is not required because it is built by compiling existing VCF files (and a VCF writer is available).
For querying capabilities an index of the .bcf file is required, this is a .bcf.csi file.
Testing is included and should be a good example of how to use the parser.
See ENSCORESW-3106 for more info.