[WIP] Add support for customizable formatting

[tmadlener]

BEGINRELEASENOTES

• Make it possible to specify a user defined formatting via the {u} specifier.

ENDRELEASENOTES

• Includes [WIP] Add support for formatting collections and handles #932
• Includes Make it possible to use objects and collections with fmtlib #931
• Do we need more than one customization hook? (E.g. for EDM4hep to define a better "default" formatting, but still giving the users the possibility to override)
• Test integration into EDM4hep
• Better implementation e.g. via class specialization (similar to fmt::formatter)?

This follows roughly the approach outlined in #920 (comment) and is the final piece of the chain towards customizable collection formatting.

[tmadlener]

I suppose one of the biggest questions here is whether we are OK with making fmt a hard dependency of podio for now or whether we should invest time into making a compatibility layer that switches to std::format under the hood if possible / supported by the compiler. All of this should also be possible on top of std::format, but there is no mixing & matching between the two libraries, they seem to be mutually exclusive.

The other option is to only have this functionality present in case the compiler has enough std::format support, which would put a rather high floor on the compilers. I think for most of the things we would need at least gcc15 (not sure about clang). However, that would probably also mean that we have to maintain dedicated / gated operator<< implementations for the builds without std::format support.