Allow binary-to-text conversion to be moved to a background thread

[swebb2066]

This PR provides an option for LOG4CXX_XXXXX_FMT_ASYNC macros when Log4cxx is build with fmt::fmt as a dependency

[rm5248]

For the other macros, we have the ability to use either std::format or fmt::format just by changing the namespace. Is that possible with this as well?

[swebb2066]

Is that possible with this as well?

No . The standard library has no equivalent to fmt::dynamic_format_arg_store

[rm5248]

Is that possible with this as well?

No . The standard library has no equivalent to fmt::dynamic_format_arg_store

I haven't had a chance to try this out yet, but there is std::make_format_args which may be able to be used here.

[swebb2066]

is std::make_format_args which may be able to be used here

The notes section indicates it is not an option

A formatting argument has reference semantics for user-defined types and does not extend the lifetime of args. It is the programmer's responsibility to ensure that args outlive the return value