feat(bedrock): replace PrepareAgent with shouldPrepareAgent prop

[jstrunk]

Per discussions with @massi-ang and the CloudFormation team, there shouldn't be a resource to call PrepareAgent as it doesn't create anything. This change replaces the custom resource with an optional property called shouldPrepareAgent that calls and waits for the agent to be prepared after creating or updating the agent, action group, or knowledge base association custom resources.

Alias creation and updates still prepare the agent first. Developers can choose whether to use shouldPrepareAgent or create an alias that tracks the latest changes to ensure that their agent is prepared. This change makes the alias custom resource use the same function to prepare the agent and wait as the others.

This change replaces the complicated changeId hashes with the updatedAt timestamps returned by the SDK in order to determine when to update the alias.

Custom Resource exceptions are reraised by the retry mechanism.

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[codecov]

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Additional details and impacted files

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