Move PublicKey to a separate crate

[mirgee]

This allows using PublicKey in the forthcoming did_key crate without importing the entire did_peer crate unnecessarily. The only reason PublicKey resided in the did_peer in the first place was because it was not used anywhere else yet.

[codecov-commenter]

Codecov Report

Merging #911 (c3a2c0b) into main (b49b5d9) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #911      +/-   ##
==========================================
- Coverage   43.37%   43.31%   -0.07%     
==========================================
  Files         438      439       +1     
  Lines       34777    34823      +46     
  Branches     7580     7589       +9     
==========================================
- Hits        15084    15082       -2     
- Misses      15231    15280      +49     
+ Partials     4462     4461       -1     

Flag	Coverage Δ
unittests-aries-vcx	43.31% <ø> (-0.07%)	⬇️

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Files Changed	Coverage Δ
public_key/src/key.rs	0.00% <ø> (ø)

... and 6 files with indirect coverage changes