Common: allow literal suffix and add tests for confkey

[f3sch]

This adds some testing to the configurable keys code, requiring its own ConfigurableParam o2::conf::test::TestParam which is only built if tests are built. Testing is done for setting/getting/serialziation/deserialization.
Also this adds accepting literal suffixes for fundamental types to the setting the values from a string, e.g. allowing for "TestParam.fValue=42.f" which previously failed (I have fallen for this already a couple of times since its is easy to overlook the error in the many log messages we produce).

[github-actions]

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[alibuild]

Error while checking build/O2/fullCI_slc9 for 9507a78 at 2025-09-15 11:36:

## sw/BUILD/O2-full-system-test-latest/log
Detected critical problem in logfile digi.log


## sw/BUILD/o2checkcode-latest/log
--
========== List of errors found ==========
++ GRERR=0
++ grep -v clang-diagnostic-error error-log.txt
++ grep ' error:'
++ GRERR=1
++ [[ 1 == 0 ]]
++ mkdir -p /sw/INSTALLROOT/85a1a241a1e7ca1211c355dbdfd832217b2b0126/slc9_x86-64/o2checkcode/1.0-local539/etc/modulefiles
++ cat
--

Full log here.