feat: allow fct and prf to be nullable. Fixes #98

[jsantell]

Description

Allows fct and prf to be nullable when consuming, and adhering to canonicalization, will not encode empty values in the resulting JWT.

Link to issue

#98

Type of change

This makes rs-ucan adhere to the spec as of 0.9.0 w/r/t prf and fct fields which MAY be undefined. This is a breaking change w/r/t some interfaces, but notably also the canonicalization.

Test plan (required)

Updated existing tests

[codecov-commenter]

Codecov Report

Merging #99 (2267bec) into main (c0bec08) will increase coverage by 0.06%.
The diff coverage is 66.66%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #99      +/-   ##
==========================================
+ Coverage   57.36%   57.43%   +0.06%     
==========================================
  Files          23       23              
  Lines         774      787      +13     
  Branches      187      191       +4     
==========================================
+ Hits          444      452       +8     
- Misses        202      204       +2     
- Partials      128      131       +3     

Impacted Files	Coverage Δ
ucan/src/chain.rs	63.80% <57.14%> (+0.34%)	⬆️
ucan/src/ipld/ucan.rs	71.11% <60.00%> (-3.25%)	⬇️
ucan/src/builder.rs	78.57% <75.00%> (-0.78%)	⬇️
ucan/src/ucan.rs	69.47% <100.00%> (ø)

[cdata]

Awesome!