DRIVER-434 Add header optimization

[dkropachev]

Summary

• add opt-in DynamoDB request header optimization through nullable Config::header_optimization
• make HeaderOptimization a strategy interface so applications can provide their own allowed-header policy
• add built-in DefaultHeaderOptimization and HeaderAllowlistOptimization implementations
• compute default allowlists from a decoupled HeaderOptimizationContext: base Alternator headers, credential headers only when credentials are configured, and User-Agent only when configured
• document the option and add tests for default, fixed allowlist, custom strategy, configured credentials, no configured credentials, and custom filtering

Testing

• make check
• make test-unit (AWSSDK was not found locally, so the AWS adapter target was not built in this environment)
• make build-integration cannot run locally because AWSSDK with the dynamodb component is not installed

Closes #19