Added reading of J2K comments

[radarhere]

#6909 added reading of JP2 comments. This adds reading of J2K comments as well.

JP2 comments are read from a "Contiguous codestream box" that "contains a valid and complete JPEG 2000 codestream"

Pillow/src/PIL/Jpeg2KImagePlugin.py

Lines 264 to 265 in 0e3f51d

	if self.fp.read(12).endswith(b"jp2c\xff\x4f\xff\x51"):
	self._parse_comment()

and starts by skipping hdr.

Pillow/src/PIL/Jpeg2KImagePlugin.py

Lines 298 to 301 in 0e3f51d

	def _parse_comment(self) -> None:
	hdr = self.fp.read(2)
	length = _binary.i16be(hdr)
	self.fp.seek(length - 2, os.SEEK_CUR)

Reading from a J2K image,

Pillow/src/PIL/Jpeg2KImagePlugin.py

Lines 252 to 254 in 0e3f51d

	if sig == b"\xff\x4f\xff\x51":
	self.codec = "j2k"
	self._size, self._mode = _parse_codestream(self.fp)

the hdr is already read.

Pillow/src/PIL/Jpeg2KImagePlugin.py

Lines 101 to 107 in 0e3f51d

	def _parse_codestream(fp: IO[bytes]) -> tuple[tuple[int, int], str]:
	"""Parse the JPEG 2000 codestream to extract the size and component
	count from the SIZ marker segment, returning a PIL (size, mode) tuple."""
	hdr = fp.read(2)
	lsiz = _binary.i16be(hdr)
	siz = hdr + fp.read(lsiz - 2)

So this PR moves the hdr handling out of _parse_comment.

[hugovk]

#6909 added reading of JP2 comments. This adds reading of J2K comments as well.

Let's also add release notes like that one did.