Iterating on Negotiable Quote GraphQL schema design

[DrewML]

This PR builds on the great work done by @G-Arvind in #381.

Notes:

• Few TODOs left in here, but the majority of things are complete and ready to review
• I've tried to leave thorough notes wherever things seemed ambiguous for a dev that implementing this
• This does not cover Negotiable Quote checkout, which will be covered in a separate story
• Some payment and shipping types have been removed from the original design, because B2B doesn't deal with payment types until checkout process (see previous note). They'll be added back in the Negotiable Quote checkout story
• We don't have REST API coverage for the "History" feature of negotiable quotes, so I had to read the UI code and work backwards. Would appreciate extra scrutiny of those types

For reviewers, here's some resources that helped me if you're as unfamiliar with this feature as I was starting out 😄

• Walk-Through Video of Feature (negotiable quotes starts at 15:21)
• Negotiable Quote Lifecycle Diagrams from DevDocs
• Negotiable Quote REST APIs (Note: The docs don't distinguish between APIs for buyer actions and seller actions, so be careful):
  • Manage
  • Update
• Overview from Buyer's perspective of how process works once negotiable quote is opened, including screenshots
• Overview from Buyer's perspective of how creating negotiable quote works, with screenshots

[DrewML]

Without usage examples (use cases for queries and mutations) it is difficult to say if current schema is covering all the use cases and if anything is missing.

I can add those for the 2 queries, but every mutation has a link above it to the user docs for the feature.

[nrkapoor]

@paliarush @DrewML can we merge the PR?