feat(guardrails): add Contextual grounding

[krokoko]

Fixes #656

• Update CDK min version to 2.154.1 to support new feature
• Add contextual grounding support

Pending testing

guardrails.addContextualGroundingPolicyConfig([
      {
        threshold: 0.5,
        filtersConfigType: bedrock.ContextualGroundingFilterConfigType.GROUNDING,
      },
      {
        threshold: 0.9,
        filtersConfigType: bedrock.ContextualGroundingFilterConfigType.RELEVANCE,
      },
    ]);

Agent has guardrails attached:

When asking a question with a response factually correct:

Examples when blocking:

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[laithalsaadoon]

LGTM