Enable passing of requestMetadata to the Bedrock Converse API.

[StuartLox]

Currently, SpringAI does not expose a way to pass custom requestMetadata to the Bedrock Converse API.

AWS Bedrock allows including requestMetadata in requests so that additional contextual information can appear in Bedrock audit logs in CloudWatch.

This is particularly useful for tracing requests, where correlation IDs, user session IDs, or other operational tags are required for audit and compliance purposes.

See RequestMetadata in Converse API here: https://docs.aws.amazon.com/bedrock/latest/APIReference/API_runtime_Converse.html