(aws-support-mcp-server): Add support for service-specific form fields (e.g., Region, Resource ARNs)

[AB-DBMC]

Describe the feature

These structured fields help AWS Support triage cases faster. Currently, the create_support_case
tool only supports the standard AWS Support API parameters and does not expose these service-specific
metadata fields.

Currently, all this information must be included in the communication_body as free text,
which is less structured and may not be processed the same way by AWS Support systems.

Use Case

When creating support cases via the AWS Console, users can fill out "Help us resolve your case faster"
fields that are service-specific. For example, EC2 Image Builder has:

• Region
• Pipeline name
• Recipe ARN
• Component ARN
• Error

Proposed Solution

Add optional parameters to create_support_case that allow passing service-specific metadata.
This could be implemented as:

1. A generic additional_fields parameter accepting key-value pairs
2. Or service-specific parameter sets

Other Information

AWS Support API Reference: I'm unsure if the underlying AWS Support API supports these
fields or if they are GUI-only features. Investigation needed.

Example use case:

create_support_case(
    subject="EC2 Image Builder component issue",
    service_code="service-ec2-image-builder-linux",
    category_code="components",
    severity_code="normal",
    communication_body="...",
    # NEW: Service-specific fields
    additional_context={
        "region": "eu-central-1",
        "component_arn": "arn:aws:imagebuilder:...",
        "error": "Symlink creation failed"
    }
)

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change