Add ObjectIdSerializer

Add ObjectIdSerializer

This is based on the ObjectIdSerialization class written by Shawn Pearce
for the Gerrit Code Review project in 2009 [1]. As mentioned in the
commit message there, it should be part of core JGit.

This implementation is slightly different to Shawn's version. Rather
than having separate methods for null/non-null ids, single methods are
implemented with @Nullable annotations.

[1] https://gerrit-review.googlesource.com/c/gerrit/+/9792

Change-Id: I7599cf8bd1ecd546e2252783d6d672eb76804060
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>