sdk,cli: expose contracts info in SDK and provide /contracts endpoint in CLI

[andrevmatos]

Part of #1692

Short description
Implements /contracts endpoint. Still undocumented on spec, but offered by python API.

Definition of Done

• Steps to manually test the change have been documented
• Acceptance criteria are met
• Change has been manually tested by the reviewer (dApp)

Steps to manually test the change (dApp)

[codecov]

Codecov Report

Merging #1913 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1913   +/-   ##
=======================================
  Coverage   95.61%   95.61%           
=======================================
  Files         155      155           
  Lines        5796     5798    +2     
  Branches     1089     1090    +1     
=======================================
+ Hits         5542     5544    +2     
  Misses        196      196           
  Partials       58       58           

Flag	Coverage Δ
#dapp	91.76% <ø> (ø)
#sdk	97.21% <100.00%> (+<0.01%)	⬆️
#sdk_e2e	65.53% <100.00%> (+0.06%)	⬆️
#sdk_unit	86.40% <0.00%> (-0.05%)	⬇️

Impacted Files	Coverage Δ
raiden-ts/src/raiden.ts	91.52% <100.00%> (+0.04%)	⬆️

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[weilbith]

LGTM ❤️