feat(op): Server interface

[muhlemmer]

This PR bring the new Server interface as described in #440 and adds:

• op.Server interface;
• op.webServer: a HTTP handler taking care of routing and request parsing and validating required request fields.
• op.LegacyServer: a wrapper around the current OpenIDProvider and Storage interfaces.

The interface should be considered experimental.

The following tests have been provided:

• Unit tests for op.webServer (near 100%)
• The op router test has been duplicated and runs against the LegacyServer. Some test error outcomes are different, due to the different order of things being validated. (For example client verification is done in a more centralized manner as the old implementation).
• The integration tests run two times, unmodified, against the op.Provider and op.LegacyServer

[codecov]

Codecov Report

Merging #447 (a1a6c19) into next (daf82a5) will increase coverage by 4.23%.
The diff coverage is 76.13%.

@@            Coverage Diff             @@
##             next     #447      +/-   ##
==========================================
+ Coverage   54.15%   58.39%   +4.23%     
==========================================
  Files          75       78       +3     
  Lines        5724     6542     +818     
==========================================
+ Hits         3100     3820     +720     
- Misses       2369     2426      +57     
- Partials      255      296      +41     

Files	Coverage Δ
pkg/op/client.go	79.03% <ø> (ø)
pkg/op/config.go	94.54% <ø> (ø)
pkg/op/discovery.go	98.42% <100.00%> (+0.28%)	⬆️
pkg/op/endpoint.go	100.00% <100.00%> (ø)
pkg/op/probes.go	68.18% <100.00%> (ø)
pkg/op/server_http.go	99.73% <99.73%> (ø)
pkg/op/token_code.go	39.75% <0.00%> (+3.61%)	⬆️
pkg/op/error.go	93.47% <94.44%> (+1.37%)	⬆️
pkg/op/token_request.go	60.60% <0.00%> (+1.01%)	⬆️
pkg/op/token_revocation.go	31.93% <0.00%> (-0.83%)	⬇️
... and 7 more

... and 6 files with indirect coverage changes

[livio-a]

i also found some flakiness in the tests

[github-actions]

🎉 This PR is included in version 3.0.0-next.11 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[github-actions]

🎉 This PR is included in version 3.0.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀