[POC] add free handler to the proxy lib

[bratpiorka]

[POC] Add free handler to the proxy lib

This PR:

• adds new API call umfSetProxyLibHandlerFreePre to the Proxy Lib which set the user's callback called before the Proxy Lib calls free()
• moves proxy_lib.h to the public API so it is now installed with other headers
• moves Proxy Lib tests to separate test folder
• adds simple test for umfSetProxyLibHandlerFreePre

NOTE: the baseline is v0.10.x as it contains IPC handler

TODO:

• Documentation
• Other handlers

Checklist

• Code compiles without errors locally
• All tests pass locally
• CI workflows execute properly
• New tests added, especially if they will fail without my changes
• Extended the README/documentation
• All newly added source files have a license
• All newly added source files are referenced in CMake files
• Logger (with debug/info/... messages) is used

[vinser52]

I think we need to discuss the following question at the UMF technical meeting: Should this API be part of the proxy library or the core UMF library.