[Logs SDK] Log Appender for spdlog

[chusitoo]

Implemented as described in https://github.com/open-telemetry/opentelemetry-cpp/issues/2046:

• Sink implemented in spdlog::sinks namespace for commonality with existing sinks.
• 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.