Add support for remaining config variables from the old to-be-deprecated Config interface and add a new config class

[ubaskota]

Issue #, if available:

Description of changes:
Adds the remaining AWS-shared config fields to AsyncAwsConfig, bringing it to parity with the generated service Config class.

• New fields: Adds support for endpoint_uri, aws_access_key_id, aws_secret_access_key, aws_session_token, sdk_ua_app_id, user_agent_extra, interceptors, http_request_config, transport, retry_strategy, aws_credentials_identity_resolver. Resolvable fields wire into the env > profile > default resolution pipeline.
• Service-specific codegen: Generates Async<ServiceId>Config(AsyncAwsConfig) with service-specific _FIELDS that override the base class example: endpoint_uri uses a service-aware resolver that checks AWS_ENDPOINT_URL_<SERVICE_ID> and the services config section before falling back to global sources.
• Dual config support: The generated config module now contains both the old Config (with a deprecation warning) and the new Async<ServiceId>Config, so existing users continue to work while new users adopt the async resolution path. The generated client accepts either type via isinstance dispatch.
• Supporting changes: Adds get_service_config() on MergedConfig for services-section lookups, and updates RetryStrategyResolver to accept retry_mode/max_attempts fallbacks from the config layer.

Testing:

• Added unit tests for:
  • EndpointUriResolver covering the full precedence chain: service-specific env var > global env var > service config section > global profile > unset.
  • MergedConfig.get_service_config() covering all lookup paths (profile missing, services key missing, service section not found, multiple services).
  • RetryStrategyResolver fallback behavior: retry_mode/max_attempts params used when retry_strategy is None, explicit strategy takes precedence over fallbacks.

Example Usage:

Resolve service config and inspect provenance:

# With AWS_REGION=us-east-1 set in the environment
# and ~/.aws/config containing:
#   [profile default]
#   services = my-services
#
#   [services my-services]
#   bedrock_runtime =
#     endpoint_url = https://bedrock-runtime.us-east-1.amazonaws.com

import asyncio
from aws_sdk_bedrock_runtime.config import AsyncBedrockRuntimeConfig

async def main():
  config = await AsyncBedrockRuntimeConfig.resolve()

  print(config.region)                    # "us-east-1"
  print(config.source_of("region"))       # ENV
  print(config.endpoint_uri)              # "https://bedrock-runtime.us-east-1.amazonaws.com"
  print(config.source_of("endpoint_uri")) # PROFILE

asyncio.run(main())

Invalid profile raises a clear error:

import asyncio
from aws_sdk_bedrock_runtime.config import AsyncBedrockRuntimeConfig

async def main():
  config = await AsyncBedrockRuntimeConfig.resolve(profile="non-existent")
  # raises ProfileNotFoundError:
  #   Profile 'non-existent' (from the profile argument) not found in config file.

asyncio.run(main())

Refer to #751 for more examples.

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