Initial Exif and XML metadata support for JPEG XL

[ssh4net]

Description

Initial Exif and XML metadata support for JPEG XL.
EXIF - works and enabled by default
XML - works and enabled by default
IPTC - that part need some help. JPEG XL support XML based IPTC, but I do not get what IPTC create an OIIO
JBIG - not sure

new options:
jpegxl:use_boxes - export EXIF, XMP, IPTC, JBIG
jpegxl:compress_boxes - compress meta boxes (disabled for this moment by default)

jpegxl:exif_box = 1 (enabled)
jpegxl:xmp_box = 1 (enabled)
jpegxl:jumb_box = 0 (disabled)
jpegxl:iptc_box = 0 (disabled)

Tests

Not yet, but if we have I can add.

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• If I added or modified a C++ API call, I have also amended the
  corresponding Python bindings (and if altering ImageBufAlgo functions, also
  exposed the new functionality as oiiotool options).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format before submitting, I definitely will look at the CI
  test that runs clang-format and fix anything that it highlights as being
  nonconforming.

[lgritz]

You're getting build failures on Mac because of an unused variable warning.

[lgritz]

I made a couple very minor style suggestions for clarity.

Also, there is a warning failure for clang related to not using the return value of save_metadata.

But other than those, LGTM!

Can you incorporate someting into the jxl tests so that the testsuite verifies writing and reading of metadata?

[ssh4net]

@lgritz I'll check how can add something to tests. But at this moment only writing was added.

I also need change a docs a bit

[ssh4net]

@lgritz don't know how to do a tests without implementing exif for import.
I'll probably add jpegxl import exif support, but later. Need to work on other project at this moment.

[lgritz]

@lgritz don't know how to do a tests without implementing exif for import.

No prob, we can just remember to add tests when the input support is added.

[lgritz]

@ssh4net This LGTM except for the CI failure that appears to not like 'const' in the struct definition for BoxInfo.

Also, would you mind rebasing on top of the current main just to be sure there aren't any conflicts?

After those two items, I'm good to merge this.

[lgritz]

LGTM, thanks!