updated copyright/licensing and style

[maurafortino]

What's Included:

• (unify) Use SPDX for copyright and reuse. #178
• (unify) Update code style #184

[codecov]

Codecov Report

Merging #182 (5bf355e) into main (d098c5b) will not change coverage.
Report is 8 commits behind head on main.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #182   +/-   ##
=======================================
  Coverage   78.55%   78.55%           
=======================================
  Files          35       35           
  Lines        1562     1562           
=======================================
  Hits         1227     1227           
  Misses        308      308           
  Partials       27       27           

Flag	Coverage Δ
unittests	78.55% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
client.go	0.00% <ø> (ø)
key/endpoint.go	100.00% <ø> (ø)
key/handler.go	100.00% <ø> (ø)
key/pair.go	70.00% <ø> (ø)
key/provide.go	100.00% <ø> (ø)
key/registry.go	100.00% <ø> (ø)
main.go	0.00% <ø> (ø)
metrics.go	0.00% <ø> (ø)
random/noncer.go	100.00% <ø> (ø)
random/provide.go	100.00% <ø> (ø)
... and 25 more