feat!: Custom 'now' for proof chain validation

[cdata]

This breaking change introduces an extra field when validating UCANs, on related implementation for both Ucan and ProofChain . This field lets us set the time to use when validating the UCAN with respect to its lifetime.

This is desired so that we can validate an old/probably-expired UCAN to check that an effect was allowed at the time that it occurred.

Fixes #82

[codecov-commenter]

Codecov Report

Merging #83 (0cd4464) into main (77ec26d) will increase coverage by 1.57%.
The diff coverage is 75.00%.

📣 This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
+ Coverage   55.77%   57.34%   +1.57%     
==========================================
  Files          23       23              
  Lines         762      776      +14     
  Branches      191      188       -3     
==========================================
+ Hits          425      445      +20     
+ Misses        204      202       -2     
+ Partials      133      129       -4     

Impacted Files	Coverage Δ
ucan/src/chain.rs	63.10% <66.66%> (+2.89%)	⬆️
ucan/src/ucan.rs	69.47% <88.88%> (+4.63%)	⬆️

... and 3 files with indirect coverage changes

[matheus23]

I actually realize it may be helpful to have a lower-level API that gives you an interval for when certain capabilities were valid.
A small layer around that API may then cross-check that value against now() or some other timestamp.
But it's useful to have that intermediate layer to be able to say e.g. "evict this UCAN from the cache at timestamp X" or have a UI that indicates "You'll have access for two more days".