feat: Enable the disabling of feature gates that are by default on/active

[shmuelk]

What type of PR is this?
/kind feature

What this PR does / why we need it:
As experimental features graduate to standard features, there will be those that for various reasons want to be able to disable such features. To date the feature flags are a simple list that enables all features in the list in the configuration file.

This PR does the following:

1. Extends the items in the feature flags array in the configuration to optionally be of the form gate=boolean-value where boolean-value is one of t,f,true, false,1,0 (in any case)
2. Provides the ability to specify default values for feature flags. Until now the default value for all feature flags was false. Now a default value of true can eb provided to automatically enable features.

With this PR, the graduation of the flow control feature for example, is as simple as changing:

loader.RegisterFeatureGate(flowcontrol.FeatureGate, false)

to

loader.RegisterFeatureGate(flowcontrol.FeatureGate, true)

thus changing the default value of the flow control feature gate to true.

NOTE: This PR removes the deprecated feature flag dayaLayer that was used to enable the new data layer. This feature gate was dprecated in llm-d-router 0.8.0

Which issue(s) this PR fixes:

Fixes #1680

Release note (write NONE if no user-facing change):

- The deprecated dayaLayer feature gate has been removed. It has not been needed since llm-d-router 0.8.0
- Feature gates can now be disabled in the featureGates section of the configuration file, with the syntax gate=false