Optionally allow separate configuration of auth_backends for all HTTP-based plugins (protocols, APIs)

[aaron-seo]

Proposed Changes

As discussed here #13205, and on community Slack, this PR allows a separate configuration of auth_backends for web_dispatch_access_control. This configuration is optional and designed to fallback to the regular auth_backends config, if not-configured.

Tested with the following configs (and retested with http_dispatch prefix)

Config 1 (empty):

# empty

Mgmt UI Auth with internal works, debug logs present for "fallback"

Config 2:

http_dispatch.auth_backends.1 = internal

Mgmt UI Auth with internal works, AMQP Auth with internal works, no debug logs for "fallback"

Config 3:

auth_backends.1 = ldap

Mgmt UI Auth with internal does not work, AMQP Auth with internal does not work, debug logs present for "fallback".

Config 4:

http_dispatch.auth_backends.1 = ldap

Mgmt UI Auth with internal does not work, AMQP Auth with internal works, no debug logs for "fallback"

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[michaelklishin]

@aaron-seo I'm afraid the fallback message is logged so often that it's beyond reasonable even at debug level. For example, starting a node with a management UI open and running two HTTP API client/tool test suites produces pages and pages of debug messages (one for every API request).

[michaelklishin]

Accepted with a small logging-related change in #13465