RFC 7662 Token Introspection

[0xTim]

Adds an implementation for RFC 7662 to provide a Token Introspection Endpoint for different resource servers to validate tokens with a separate server.

• [x] Middleware for a resource server to use
• [x] Integration tests to show how it would work
• [x] README

[codecov[bot]]

Codecov Report

Merging #2 into master will increase coverage by 1.71%. The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master       #2      +/-   ##
==========================================
+ Coverage   90.85%   92.57%   +1.71%     
==========================================
  Files          25       33       +8     
  Lines         711      889     +178     
==========================================
+ Hits          646      823     +177     
- Misses         65       66       +1

Impacted Files	Coverage Δ
.../TokenHandlers/ClientCredentialsTokenHandler.swift	100% <ø> (ø)
...rces/VaporOAuth/Utilities/TokenAuthenticator.swift	76.47% <ø> (ø)
Sources/VaporOAuth/Validators/CodeValidator.swift	100% <ø> (ø)
...Auth/DefaultImplementations/EmptyUserManager.swift	100% <ø> (ø)
Sources/VaporOAuth/Models/OAuthCode.swift	100% <ø> (ø)
...ultImplementations/EmptyAuthorizationHandler.swift	100% <ø> (ø)
...eHandlers/TokenHandlers/AuthCodeTokenHandler.swift	100% <ø> (ø)
Sources/VaporOAuth/Models/OAuthUser.swift	100% <ø> (ø)
Sources/VaporOAuth/Validators/ScopeValidator.swift	100% <ø> (ø)
...VaporOAuth/RouteHandlers/AuthorizeGetHandler.swift	95.5% <ø> (ø)
... and 36 more

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