dart-next fixes

[kuhnroyal]

Allow sync (de)serialization & fix some parameter bugs

• serialize path/query/form parameters synchronously
• force cast non-nullable parameters

Fix built_value collection builders in serializers

[ahmednfwela]

@kuhnroyal
my plan for the repository was to have 2 versions
ISyncRepositoryBase, IAsyncRepositoryBase, where all implementations are required to provide the ISyncRepositoryBase version.
and the IAsyncRepositoryBase can fall back to calling the ISyncRepositoryBase.

and when sending requests/receiving responses, we use the IAsyncRepositoryBase

[ahmednfwela]

the plan right now is:

1. finalize the repository base classes
2. make json_serializable work
3. add tests