Use aiobotocore for the ChatBedrock async implementation

[fabiopicchi]

Checked

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Feature request

To avoid the creation of a ThreadPoolExecutor, we should evaluate using aiobotocore for ChatBedrock. It is reasonably stable and even added support to BedrockConverse.

Motivation

Better control over the application's lifecycle. I have a use case in which I spin up a background thread that schedules langchain tasks to its own event loop (main thread runs synchronous code) and I would like to avoid the creation of another ThreadPoolExecutor from this background thread to help debuggability and tame complexity.

Proposal (If applicable)

My proposal would be to keep the _astream and other asynchronous methods from the interface but reimplement them using aiobotocore.