Add ability to bypass AA with HTTP header

[dddpaul]

For example in config:

authentication:
  configuration:
    bypass_header: X-Marathon-Plugins-Auth-Disabled

[dddpaul]

For the sake of non-breaking changes - it may be more reasonable to enable AA only if special HTTP header is exists.

For example:

authentication:
  enabled: true
  configuration:
    force_authentication_header: X-Marathon-Plugin-Auth-Force
    force_authentication_header_enabled: true

Modes:

Force header mode	Bypass header mode	Result
Off	Off	AA is performed always
Off	On	AA is performed unless bypass header was received
On	Off	AA is performed only if force header was received
On	On	AA is performed always or incorrect configuration?

[dddpaul]

I think, this is the most flexible behaviour:

authentication:
  enabled: true
  configuration:
    production_mode_enabled: true
    trigger_header: X-Marathon-Plugin-Auth-Enabled

Parameters:

• authentication.enabled (optional): Disable or enable plugin. By default plugin is enabled.
• authentication.configuration.production_mode_enabled (optional): If true, then AA is enabled, but can be disabled by trigger header being set to false. If false then AA is disabled, but can be enabled by trigger header being set to true. By default is true.
• authentication.configuration.trigger_header (optional): HTTP header name for trigger on/off AA. By default is null, in this case triggering fuctionality is disabled.

Configuration examples:

1. Disable plugin at all:

authentication:
  enabled: false

2. Enable plugin but AA is disabled for all requests except triggering ones:

authentication:
  configuration:
    production_mode_enabled: false
    trigger_header: X-Marathon-Plugin-Auth-Enabled

3. Enable plugin but AA is disabled for all requests except requests with HTTP Basic Auth:

authentication:
  configuration:
    production_mode_enabled: false
    trigger_header: Authorization

4. AA is enabled for all requests except triggering ones:

authentication:
  configuration:
    production_mode_enabled: true
    trigger_header: X-Marathon-Plugin-Auth-Enabled

5. AA is enabled for all of the requests:

authentication:
  configuration:
    production_mode_enabled: true
    trigger_header: X-Marathon-Plugin-Auth-Enabled