Add copyright notice and fix CLA workflow

[nlordell]

This PR does a few things:

• It adds a COPYRIGHT.md notice to our repo.
• Relicenses the testlib to be MIT (matches our other "library" crates).
• Fixes the CLA workflow. There were some differences in the private repo I tested with and this public repo.

Test Plan

Check the CLA bot working in another public repo: https://github.com/cowprotocol/hdnode/runs/6194783177?check_suite_focus=true for cowprotocol/hdnode#7

Also check that the CLA workflow matches the one from the instructions https://github.com/cowprotocol/cla#github-action-setup

[codecov-commenter]

Codecov Report

Merging #174 (6adc836) into main (74468af) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #174      +/-   ##
==========================================
- Coverage   64.82%   64.82%   -0.01%     
==========================================
  Files         185      185              
  Lines       38398    38398              
==========================================
- Hits        24892    24890       -2     
- Misses      13506    13508       +2