Adds 6 public methods on FLAC::File mirroring RIFF::WAV::File's existing
iXML/BEXT API: iXMLData/setiXMLData/hasiXMLData and the BEXT equivalents.
Reads APPLICATION blocks (RFC 9639 § 8.4) carrying either the IANA-
registered "riff" foreign-metadata wrapper or the direct "iXML" / "bext"
application IDs used by some third-party tools (e.g. Sequoia). Writes
the spec-blessed "riff"-wrapped form. Unrecognized application IDs and
"riff"-wrapped chunks other than iXML/bext (e.g. "fmt ", "JUNK") flow
through unmodified, so existing files round-trip without churn.
Test coverage: read direct + riff-wrapped for both iXML and BEXT,
write+reread round-trip, empty-clears-block, and an unknown-application-
block preservation guard.