auth: add www-authenticate header

[System-Glitch]

References

Issue(s): #278

Description

• Added SchemeAuthenticator interface that authenticators can implement to indicate the scheme they are using.
• On authentication failure, the auth middleware adds the WWW-Authenticate header to the response if the authenticator implements SchemeAuthenticator.
• Added auth.MiddlewareWithRealm to provide custom realm description.
• Using this approach instead of implementing OnAunthorizer on existing authenticator implementations avoids breaking custom implementations that use composition with the built-in ones.

Resources:

• https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/WWW-Authenticate
• https://www.iana.org/assignments/http-authschemes/http-authschemes.xhtml

Possible drawbacks

None.

[coveralls]

Pull Request Test Coverage Report for Build 18871375303

Details

• 25 of 25 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.009%) to 97.545%

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Totals
Change from base Build 18378152916:	0.009%
Covered Lines:	6675
Relevant Lines:	6843

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💛 - Coveralls