avro

add two cmake options to make it easier to integrate avro c++ (#3268)

AVRO_BUILD_EXECUTABLES controls whether to build precompile and avrogencpp
AVRO_BUILD_TESTS controls whether to build unittest

The two options default to ON conforming the original behaviour.

This is needed because for libraries like iceberg-cpp, we don't need
to build the unittest and executable.

Signed-off-by: Junwang Zhao <zhjwpku@gmail.com>