Add RNA read phasing source

[iskandr]

Summary

• add RNAReadPhasingSource, a BAM-backed ReadPhasingSource consumed through ReadPhaseResolver(source)
• support SNV/MNV, insertion, and deletion allele calls from quality-filtered RNA reads/fragments, including paired-end fragment co-occurrence
• add varcode[rna] optional pysam extra, public export, docs, changelog, and 5.0.1 version bump

Tests

• ./lint.sh
• ./test.sh (1204 passed, 2 skipped)

Fixes #354