API key authentication implementation

[noavarice]

See GH-17563

Adds API key authentication support. Key components include the following:

• ApiKei is a data model consisting of ID and secret parts. ApiKey provides method for generating new instance randomly;
• ApiKeyDigest handles hashing API key secret part and comparing secret with existing hash. This component closely resembles PasswordEncoder functionality;
• ApiKeySearchService allows framework user to hook into the API key authentication lifecycle. This component closely resembles UserDetailsService functionality;
• ApiKeyAuthenticationProvider handles API key authentication aspects;
• ApiKeyAuthenticationFilter intercepts incoming requests and hands control over to AuthenticationManager the same way other similar filters do (e.g., BasicAuthenticationFilter).

This is a work-in-progress, it's far from finish (no reactive stack support, no tests, etc.), but it catches the way it should work from my perspective.