REVIEW: [NEXUS-5490] Extensible authentication token factories & nexus-rutauth-plugin

https://issues.sonatype.org/browse/NEXUS-5490
https://bamboo.zion.sonatype.com/browse/NX-OSSF10

Introduces NexusAuthenticationFilter that is using an pluggable list of AuthenticationTokenFactory to create authentication tokens based on http request/response

The filter will first ask auth. token factories to create an authentication token based on http request/response. First one that returns an auth. token will win. if none of them will be able to create an auth. token will fall back to NexusSecureHttpAuthenticationFilter (like current setup)

The new filter is bounded to authcBasic/authcNxBasic as a replacement of NexusSecureHttpAuthenticationFilter.

The new filter can replace NexusApiKeyAuthenticationFilter/NexusApiKey with minimal changes in nuget plugin