[v2] Add support for configured manual auth scheme and bearer token auth

[hssyoo]

Ports boto/botocore@e714dec and boto/botocore#3526

This PR adds support for configuring preferred auth schemes and also service-specific bearer token auth for Bedrock. This allows users to pass a bearer token to authenticate Bedrock requests.

To manually test:

1. Generate Bedrock bearer token
2. Set env vars:

export AWS_BEARER_TOKEN_BEDROCK=$GENERATED_TOKEN
export AWS_AUTH_SCHEME_PREFERENCE=httpBearerAuth

3. Run aws bedrock list-prompt-routers --debug

Wire level request should use bearer token and register 3 for feature id:

Sending http request: <AWSPreparedRequest stream_output=False,
method=GET, url=https://bedrock.us-west-2.amazonaws.com/prompt-routers,
headers={'User-Agent': b'aws-cli/2.27.61 md/awscrt#0.26.1 ua/2.1
lang/python#3.13.4 md/pyimpl#CPython m/Z,E,C,b,3 cfg/retry-mode#standard
md/installer#source sid/cf1d50a2a093 md/prompt#off
md/command#bedrock.list-prompt-routers',
'Authorization': b'Bearer bedrock-api-key-MY-REDACED-KEY'}>