Adjust modifiers and add some properties to RaidenChannel & RaidenTransfer interfaces

[andrevmatos]

Fixes #1657
Fixes #1708

Short description
RaidenChannel's id, settleTimeout and openBlock are marked as required and therefore always defined
RaidenTransfer's secret is added as optional property, and'll hold transfer's secret whenever we know it (usually true for sent transfers)

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)

1. Nothing to test, mostly types and public data adjustments

[github-actions]

You modified raiden-dapp/src,
Please remember to add a change log entry at raiden-dapp/CHANGELOG.md if necessary.

[codecov]

Codecov Report

Merging #1722 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1722   +/-   ##
=======================================
  Coverage   95.30%   95.31%           
=======================================
  Files         148      148           
  Lines        5330     5331    +1     
  Branches      968     1025   +57     
=======================================
+ Hits         5080     5081    +1     
  Misses        202      202           
  Partials       48       48           

Flag	Coverage Δ
#dapp	91.48% <100.00%> (ø)
#sdk	96.86% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
raiden-ts/src/channels/state.ts	100.00% <ø> (ø)
raiden-ts/src/transfers/state.ts	100.00% <ø> (ø)
raiden-dapp/src/components/navigation/Channels.vue	100.00% <100.00%> (ø)
raiden-ts/src/transfers/utils.ts	93.42% <100.00%> (+0.08%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cc2d9ca...6153624. Read the comment docs.

[weilbith]

Thanks for doing this so quick. Really helps me out! 🙏

[weilbith]

If we do this quick. I can even integrate this into #1711 already and remove the issue reference.