Add support for environment token provider for services whose SigV4 signing...

Add support for environment token provider for services whose SigV4 signing names are `bedrock` (#4241)

## Motivation and Context
Adds support for environment token provider for AWS services whose SigV4
service signing name matches `bedrock`. Setting this environment
variable, `AWS_BEARER_TOKEN_BEDROCK`, allows SDKs to prefer the
`httpBearerAuth` auth scheme and to retrieve a `Token` value from the
said environment.

## Description
Customers would use the environment variable in question like so:
```
    // export AWS_BEARER_TOKEN_BEDROCK=my-token

    let sdk_config = aws_config::defaults(BehaviorVersion::latest()).load().await;

    let bedrock_client = aws_sdk_bedrock::Client::new(&sdk_config);
    // call an operation on `bedrock_client`...
```
Under the hood, this is equivalent roughly to
```
    let sdk_config = aws_config::defaults(BehaviorVersion::latest()).load().await;
    let bedrock_config = aws_sdk_bedrock::config::Builder::from(sdk_config)
        .auth_scheme_preference([HTTP_BEARER_AUTH_SCHEME_ID])
        .token_provider(Token::new("my-token", None))
        .build();

    let bedrock_client = aws_sdk_bedrock::Client::from_conf(bedrock_config);
    // call an operation on `bedrock_client`...
```
This behind-the-scenes convenience is implemented in `impl
From<&SdkConfig> for Builder`, similar to how a service-specific
environment is implemented for the [endpoint
URL](https://docs.aws.amazon.com/sdkref/latest/guide/feature-ss-endpoints.html#ss-endpoints-envar).

However, `impl From<&SdkConfig> for Builder` implies that customers need
to create a service client from `SdkConfig` (typically through
[ConfigLoader::load](https://docs.rs/aws-config/latest/aws_config/struct.ConfigLoader.html#method.load))
in order to take advantage of the environment variable. If customers
create the service client directly from the service config builder, the
environment variable will not be applied, i.e.
```
    // export AWS_BEARER_TOKEN_BEDROCK=my-token

    let bedrock_config = aws_sdk_bedrock::Config::builder()
        // other configurations
        .build();

    let bedrock_client = aws_sdk_bedrock::Client::from_conf(bedrock_config);
    // `bedrock_client` neither prefers HTTP_BEARER_AUTH_SCHEME_ID nor sets a Token with my-token.
```

## Testing
- Added integration tests for `bedrockruntime` (whose model is already
checked in to `aws/sdk/aws-models`)

## Checklist
- [x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

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