WIP: GOBL Net verify and certify process

[samlown]

• Starting an initial process for verification of signers using JSON Web Key Set lookups.
• Moves some of the logic of signing and verification in Envelope to the Header.
• New net package defines all the logic to handle lookups of public keys to the GOBL host name plus /.well-known/gobl/jwks.json.
• Supports chained signing, so that the jwks.json itself can be signed by another authority, such as GOBL Net itself.

Pre-Review Checklist

• Opened this PR as a draft
• Read the CONTRIBUTING.md guide.
• Performed a self-review of my code.
• Added thorough tests with at least 90% code coverage.
• Modified or created example GOBL documents to show my changes in use, if appropriate.
• Added links to the source of the changes in tax regimes or addons, either structured or in the comments.
• Run go generate . to ensure that the Schemas and Regime data are up to date.
• Reviewed and fixed all linter warnings.
• Been obsessive with pointer nil checks to avoid panics.
• Updated the CHANGELOG.md with an overview of my changes.
• Marked this PR as ready for review.

And if you are part of the org:

• Requested a review from Copilot and fixed or dismissed (with a reason) all the feedback raised.
• Requested a review from @samlown.

[codecov]

Codecov Report

❌ Patch coverage is 58.71212% with 109 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.64%. Comparing base (72beee4) to head (0336766).

Files with missing lines	Patch %	Lines
head/header.go	0.00%	29 Missing ⚠️
net/keyset.go	71.64%	11 Missing and 8 partials ⚠️
net/client.go	65.30%	16 Missing and 1 partial ⚠️
net/verify.go	73.58%	7 Missing and 7 partials ⚠️
internal/cli/verify.go	0.00%	12 Missing ⚠️
cmd/gobl/verify.go	22.22%	6 Missing and 1 partial ⚠️
dsig/key.go	0.00%	5 Missing ⚠️
dsig/signature.go	66.66%	2 Missing and 2 partials ⚠️
net/authorities.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #785      +/-   ##
==========================================
- Coverage   93.09%   92.64%   -0.46%     
==========================================
  Files         368      373       +5     
  Lines       19624    19871     +247     
==========================================
+ Hits        18269    18409     +140     
- Misses        919     1008      +89     
- Partials      436      454      +18     

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