Add fmtlib dependency

[feltech]

Description

Closes #1070.

fmtlib allows for easy, safe string substitution in C++, adding advanced formatting functionality (e.g. rounding), and reducing bloat.

So add as a new private build-only header-only dependency.

Using header-only increases compile times, but subjectively it's not noticeable, and has the benefit of avoiding a possible public library dependency (if OpenAssetIO is built as a static library).

Tweak a single existing, tested, log string to make use of libfmt, to ensure basic usage is validated.

Added to openassetio-build Docker image: https://github.com/OpenAssetIO/OpenAssetIO/pkgs/container/openassetio-build

• I have updated the release notes.
• [ ] I have updated all relevant user documentation.

Reviewer Notes

Test Instructions

[foundrytom]

I know we removed CONDING_STANDARDS as there is only one way to use the header version - but its it worth adding a more general note that "we use libfmt for string building wherever needed, do not use stringstream etc..."

[feltech]

Added an emoji to the release notes to signal a build dependency, see what you think (open to ideas on alternative emojis).

I know we removed CONDING_STANDARDS as there is only one way to use the header version - but its it worth adding a more general note that "we use libfmt for string building wherever needed, do not use stringstream etc..."

Added (force-pushed).

[feltech]

Fixup pushed to reduce fmtlib version to 9.1.0, which doesn't export errant symbols. Also included the Makefile change for the Docker container.

[foundrytom]

Fixup pushed to reduce fmtlib version to 9.1.0, which doesn't export errant symbols. Also included the Makefile change for the Docker container.

Thanks for posting upstream does this comment imply we may have issues on Windows with the older version?

Potential symbol visibility issues

[feltech]

Fixup pushed to reduce fmtlib version to 9.1.0, which doesn't export errant symbols. Also included the Makefile change for the Docker container.

Thanks for posting upstream does this comment imply we may have issues on Windows with the older version?

As discussed offline - posted a follow-up question upstream to make sure I understand, but pretty certain we don't have a problem, especially in header-only mode.

[foundrytom]

Assuming were happy with the symbol situation, then LGTM 👍 🙏