fw-update: simplify PackageParser class

Since neither libpldm nor libpldm++ require separate codes for each
package format revision, only one implementation is required.

libpldm can parse any pldm fw update package where the pin passed to it
is greater or equal to the revision of that package.

See [1] for an example of libpldm parsing a package of a lower revision
than the pin passed to it.

Also, due to the spec not having removed or significantly changed
any of the old fields inside the package in newer revisions, it is
possible to parse packages of different revision into the same struct,
using optional/vector/map/... types for newly added members which may
not be present in older package format versions. See for example [2].

This simplifies the class considerably.

Tested: covered by existing unit tests for PackageParser

References:
[1] https://github.com/openbmc/libpldm/blob/4aea3a69199cb780256f0f06b05c9e0db14f6044/tests/unit/dsp/firmware_update.cpp#L5641-L5681
[2] https://github.com/openbmc/libpldm/blob/84ef0f40e8f5565785ef0b97cb0352ffdb33c2bc/include/libpldm%2B%2B/firmware_update.hpp#L225

Change-Id: I6a0c0c4e78acbac60dd1e7bf9e616dc70050376e
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>