[BEAM-2021] Convert Coder into an Abstract Static Class

[tgroh]

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[tgroh]

R: @kennknowles @dhalperi

Should be API compatible for all users who do not implement Coder directly, which should be a very small amount.

It is worth considering also making the constructor for Coder package-private and directing users to implement (Structured/Atomic/Custom)Coder

This is the minimal backwards-incompatible change for #2829 that will require a worker dance.

[dhalperi]

This LGTM

[tgroh]

Had to also update the CoderFactories to be able to resolve the parent Coder type.

[dhalperi]

Still LGTM

[tgroh]

Updated worker version for new worker that is compatible.

[coveralls]

Coverage increased (+0.006%) to 70.495% when pulling f4fa283 on tgroh:coder_abstract_class into 3bffe0e on apache:master.

[coveralls]

Coverage increased (+0.02%) to 70.505% when pulling f4fa283 on tgroh:coder_abstract_class into 3bffe0e on apache:master.