support api key authentication for Bedrock API (#536)

AWS Bedrock has introduced API key support on 7/7, simplifying
authentication for the Amazon Bedrock API. To enable this new feature in
langchain-aws, this update bumps the required boto3 version to
"^1.39.0". It has been tested and worked as expected with new
`AWS_BEARER_TOKEN_BEDROCK` environment variable.


- [x] bump up the version of boto3 in pyproject.toml and update the
poetry.lock
- [x] update README file to add API key support feature for Bedrock.

Author: 0x-fang

libs/aws/README.md

@@ -9,6 +9,11 @@ pip install -U langchain-aws
 ```
 All integrations in this package assume that you have the credentials setup to connect with AWS services.
 
+## Authentication
+
+In order to use Amazon Bedrock models, you need to configure AWS credentials. One of the options is to set the `AWS_ACCESS_KEY_ID` and `AWS_SECRET_ACCESS_KEY` environment variables. More information can be found [here](https://docs.aws.amazon.com/bedrock/latest/userguide/security-iam.html). 
+Alternatively, set the `AWS_BEARER_TOKEN_BEDROCK` environment variable locally for API Key authentication. For additional API key details, refer to [docs](https://docs.aws.amazon.com/bedrock/latest/userguide/api-keys.html).
+
 ## Chat Models
 
 `ChatBedrock` class exposes chat models from Bedrock.

libs/aws/poetry.lock

@@ -13,7 +13,7 @@ license = "MIT"
 [tool.poetry.dependencies]
 python = ">=3.9"
 langchain-core = "^0.3.66"
-boto3 = ">=1.37.24"
+boto3 = ">=1.39.0"
 pydantic = ">=2.10.0,<3"
 numpy = [
   { version = "^1", python = "<3.12" },