[CONFIGURATION] Implement ComposableSampler Extensions

[marcalff]

In the yaml schema spec:

  ExperimentalComposableSampler:
    type: object
    additionalProperties: <------------------------------------------------------------------- HERE
      type:
        - object
        - "null"
    minProperties: 1
    maxProperties: 1
    properties:
      always_off:
        $ref: "#/$defs/ExperimentalComposableAlwaysOffSampler"
        description: Configure sampler to be always_off.
        defaultBehavior: ignore
      always_on:
        $ref: "#/$defs/ExperimentalComposableAlwaysOnSampler"
        description: Configure sampler to be always_on.
        defaultBehavior: ignore
      parent_threshold:
        $ref: "#/$defs/ExperimentalComposableParentThresholdSampler"
        description: |
          Configure sampler to be parent_threshold.
        defaultBehavior: ignore
      probability:
        $ref: "#/$defs/ExperimentalComposableProbabilitySampler"
        description: Configure sampler to be probability.
        defaultBehavior: ignore
      rule_based:
        $ref: "#/$defs/ExperimentalComposableRuleBasedSampler"
        description: Configure sampler to be rule_based.
        defaultBehavior: ignore

composable samplers can ALSO be an extension point (additionalProperties).

This was missed when fixing #4028.

Applications should be allowed to add a custom composable sampler.

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