urn: add URN parser and helpers

[hdonnay]

This sets up some support infrastructure to unify naming across claircore. The goal is to be able to pass around names as strings instead of interface objects.

I'm unsure about the Name type -- thoughts on if that should live here and if it should get a method on URN?

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.00000% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 56.01%. Comparing base (e8f9aff) to head (e26ece1).

Files	Patch %	Lines
toolkit/urn/urn.go	87.32%	5 Missing and 4 partials :warning:
toolkit/urn/escape.go	92.59%	1 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1323      +/-   ##
==========================================
+ Coverage   55.76%   56.01%   +0.24%     
==========================================
  Files         265      268       +3     
  Lines       16562    16672     +110     
==========================================
+ Hits         9236     9339     +103     
- Misses       6365     6369       +4     
- Partials      961      964       +3     

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[hdonnay]

Re-drafting this until I figure out what to do about percent-encoding.