feat: request, make synced_bcf_reader::bcf_sr_add_reader equivalent that accepts htsFile

[brentp]

The header has int bcf_sr_add_reader(bcf_srs_t *files, const char *fname)

It would be nice to have something like: int bcf_sr_add_hreader(bcf_srs_t *files, htsFile *file_ptr)

My used case is code that is detecting the file type (BAM/CRAM/BCF/etc) using an htsFile and then I want to still use bcf_synced_reader if it is a BCF/VCF.

[daviesrob]

I think this should be possible. We'll look into it.