[Meta] REST-API v2 improvements

[ezdac]

Meta-issue

As users are developing against the Raiden API v1, they will have feature requests or might notice design-flaws of the current API. This meta-issue will be a place where feature requests are gathered, to allow for a high-level discussion on the next iteration of the API.

• Filtering model:
  • [ ] #6283, #6642: Better filtering for payment events
• [ ] Add a Pub/Sub or Push model for Events (Meta: #5455):
  • [ ] #4756, #6643 Allow event subscription
• Expose transport related operations:
  • [ ] #4621: Poll user presence
• Expose Blockchain related operations:
  • [ ] #5497 Expose (UDC) contract address
• Expose Raiden internal operations:
  • [ ] #6838 Expose logged warnings
• Improve Resources:
  • [ ] #5186 EventPaymentSentSuccess
  • [ ] #6368 Inconsistencies in /payments resource for different methods
  • [ ] #1202 support more flexible Metadata for transfers to allow for more direct 'in-band' communication of payment intent

Fixes (may be applicable to API v1):

• #6320: Error definition unclear
• #6516: Enable Cross Token payments
• #6137: /status inconsistencies with docs
• #5474: /pending_transfers actual payment amount missing

Internals / Performance (loosely related to public API):

• #5664 improve (de-)serialisation model