libpldm: Add pldm_entity_extract method

Add the pldm_entity_extract method in libpldm/pdr.h and extract
pldm entity structure explicitly.

Tested: built pldm daemon successfully and the unit test passes.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I330e0c76e75d01093148c3a432fcc18cea1d36e3