[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:
  • Query all payments with a specific payment partner #6283, Allow better filtering on the payments endpoint #6642: Better filtering for payment events
• Add a Pub/Sub or Push model for Events (Meta: [Meta] Add asynchronous APIs #5455):
  • SubscribeChannelEvents #4756, Subscribe to Events #6643 Allow event subscription
• Expose transport related operations:
  • API: show channel's partner state (active, inactive) #4621: Poll user presence
• Expose Blockchain related operations:
  • API endpoint for UDC contract to deposit and withdraw RDN tokens  #5497 Expose (UDC) contract address
• Expose Raiden internal operations:
  • Client notifications/warnings #6838 Expose logged warnings
• Improve Resources:
  • Add amount_for_fees to EventPaymentSentSuccess #5186 EventPaymentSentSuccess
  • RESTful API uses different representations for payment resource #6368 Inconsistencies in /payments resource for different methods
  • Payment metadata #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):

• Unclear error definitions in API specification #6320: Error definition unclear
• Add API endpoint for cross token payments #6516: Enable Cross Token payments
• /status API endpoint should return blocks to sync instead of unavailable #6137: /status inconsistencies with docs
• Pending transfers endpoint misses payment amount #5474: /pending_transfers actual payment amount missing

Internals / Performance (loosely related to public API):

• Rework (de)serialization in REST API #5664 improve (de-)serialisation model