fix(bedrock): adopt best practices

[aws-rafams]

Fixes #733

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[krokoko]

Thanks @aws-rafams ! Did a first pass and looks good to me overall. Left one comment about type aliasing.
Next steps: I will test the package in TS and Python for couple of use cases, and put back my results here
Once done, if everything works as expected, will approve

[krokoko]

First results from testing in TS:

• Agent created with guardrails (TS)

The agent is correctly created, as well as the guardrails

Guardrails are correctly working (permissions are correctly applied):

• Guardrails imported from existing resource created through the console: works but there is not much to do with it since it cannot be added to an agent

[krokoko]

Testing in Python: everything seems to work, except the GuardrailSampleTopics which don't seem to be exported

[krokoko]

Thanks @aws-rafams ! Overall it is good, I tested in both TS and Python
Fixed a readme issue and added Python code snippets

Only blocking issue is the GuardrailSampleTopics which doesn't seem to be exported correctly In Python (cannot access it). It is working fine in Ts. Once fixed, I will approve.

My other comments are non blocking, and could be some work part of a subsequent PR.

Thank you !!

[krokoko]

Tested in both Ts and Py following recent changes:

• Sample Topics are exported correctly, tested to create and add to guardrails
• Create guardrails using the constructor and attach them to an agent
• Create CfnGuardrail using the L1 CDK construct, import them using fromCfnGuardrail and attach it to an agent
• Create guardrails through the console, import them using fromGuardrailAttributes and attach it to an agent

Every scenario was working as expected. Thanks @aws-rafams , great job !