[Logs SDK] Log Appender for log4cxx

[chusitoo]

Implemented as described in open-telemetry/opentelemetry-cpp#2047:

• Appender implemented in log4cxx namespace as the macros that generate boilerplate code appear to depend on that invariant.
• Minimal examples provided to demonstrate the setup required.
• Basic unit testing.
• Tried using the same flags as those used in otel-cpp for configuring cmake project.
• Add clang format file to help keep the style in line with otel-cpp repo

[marcalff]

Thanks for the contribution.