CDP-484 add method to fetch ilk by cdp ID

[b-pmcg]

No description provided.

[b-pmcg]

see OasisDEX/mcd-cdp-portal#156

[codecov-io]

Codecov Report

Merging #190 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             dev    #190      +/-   ##
========================================
+ Coverage   92.5%   92.5%   +<.01%     
========================================
  Files         87      87              
  Lines       2441    2442       +1     
========================================
+ Hits        2258    2259       +1     
  Misses       183     183

Impacted Files	Coverage Δ
packages/dai-plugin-mcd/src/CdpManager.js	96.64% <100%> (+0.02%)	⬆️

---

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 ae1b2a0...a1c6f13. Read the comment docs.

[levity]

I'm still not sure this is necessary for OasisDEX/mcd-cdp-portal#156, but it seems like it might come in handy at some point anyway. The only thing I would suggest is renaming it to getIlkForCdp. Might seem like a trivial change, but this wording suggests to me that there is a single ilk for a cdp, which is the case, whereas getIlkByCdpId makes it sound like cdp id is a property of an ilk.