Add nonce endpoint to OID4VCI exchanges.

[dlongley]

Needs test.

[dlongley]

Addresses #120.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.11%. Comparing base (105bdb7) to head (0b210ef).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
+ Coverage   82.05%   82.11%   +0.06%     
==========================================
  Files          15       15              
  Lines        4374     4390      +16     
==========================================
+ Hits         3589     3605      +16     
  Misses        785      785              

Files with missing lines	Coverage Δ
lib/oid4/http.js	95.69% <100.00%> (+0.16%)	⬆️
lib/oid4/oid4vci.js	89.78% <100.00%> (+0.01%)	⬆️

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