Add task tags parameter (#1822)

[beckjake]

Fixes #1822

• Created a base class for parameter types

  • include arbitrary "task_tags" parameter
  • include "timeout" so we can extract/document full/valid json schemas

• Added output to ps and poll results (including errors)

• Fixed a number of type annotation issues along the way

• Refactored/moved some contracts that were scattered all over the place into contracts/rpc.py

• Added an encoder for number.Real

  • this avoids an issue with values that can be floats or ints and are therefore valid under multiple encodings.

[drewbanin]

I did some functional testing here and everything worked great!

I didn't spend so much time on the type annotation updates here. I can dig in further if that would be helpful, but if you feel confident in this as-is, then let's ship it!

[beckjake]

I think since mypy passes, all is well on the type annotations front!