feat: add code to interact with authority service

[mfontanini]

This adds an AuthorityService class that allows getting tokens from the authority service.

Example use

    key = PrivateKey()
    service = AuthorityService("http://localhost:30921")

    service_key = service.about().public_key
    envelope = service.request_token(key)
    envelope.validate_signatures()

    assert envelope.token.token.issuer.public_key == service_key

[coveralls]

Pull Request Test Coverage Report for Build 13774605068

Details

• 23 of 27 (85.19%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.5%) to 93.939%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/nuc/authority.py	23	27	85.19%

Totals
Change from base Build 13772601848:	-0.5%
Covered Lines:	465
Relevant Lines:	495

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💛 - Coveralls