Implement updated standard retry behavior

[Alan4506]

Description of changes:

Updates the standard retry behavior:

• Throttling backoff: throttling errors use a larger base backoff (1000ms) than other errors (50ms), based on error.is_throttling_error.
• Retry quota: RETRY_COST is now 14, throttling retries cost 5, and the timeout-specific cost is removed.
• DynamoDB defaults: DynamoDB / DynamoDB Streams clients default to 4 max attempts and a 25ms base backoff, applied per-value (a value configured by the customer always wins). The generic runtime stays AWS-agnostic; the DynamoDB knowledge lives in an AWS codegen integration.
• Long-polling: long-polling operations (SQS ReceiveMessage, SFN GetActivityTask, SWF PollForActivityTask/PollForDecisionTask) back off before giving up even when the quota is exhausted. Operation detection uses a per-operation codegen hook so no AWS service names leak into generic codegen.
• x-amz-retry-after: honored when returned by the service, capped at 5 seconds above the normal backoff; invalid/missing values fall back to normal backoff and the standard HTTP Retry-After is ignored.

Testing:

Added unit tests for each change (throttling vs non-throttling backoff, quota costs, DynamoDB per-value defaults, the long-polling backoff matrix, and x-amz-retry-after parsing/clamping/wiring). Also updated affected functional tests. All Python checks pass (make check-py, make test-py) and the Java codegen builds successfully.

Follow-up:

The awsJson protocol's _create_error is not on develop yet (it lives on the json-rpc branch). Once that merges, it needs the same parse_retry_after(response) wiring that awsQuery already has, so x-amz-retry-after is honored for awsJson services (e.g. DynamoDB) too.

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