Custom primary key serialization.

[ovangle]

Support custom serialization options for PrimaryKeyRelatedFields.

[xordoquy]

I'm a bit confused about this PR against #2788

[ovangle]

This PR has little to do with #2788 #3000.

Currently, the code in question assumes that the primary key's value is a primitive and passes it on to the json/xml/etc serializer to serialize. In the case of UUID's, this just calls str on the result, writing a 'verbose hex' format UUID in the serialized representation.

This may not be the desired behaviour, so I've changed the foreign key field to accept an optional subfield, which can be used to customise the output representation of the foreign key's value.

But it can be used to customise the output serialization of primary keys in general.

ps. the implementation of this should have been independent to the implementation of #2788, but the unit tests rely on being able to serialise a UUID to a format other than the verbose hex format.

[ovangle]

Rebased to use implementation from #3000.

[xordoquy]

This looks good to me. Thanks for the work.
I wonder how helpful this will be for NoSQL databases.