feat: Add SEP-973 icons and metadata support (#912)

Add Icon record and icons field to Implementation, Resource,
ResourceTemplate, Prompt, and Tool records per SEP-973. Add
websiteUrl and description fields to Implementation.

All fields are optional and backward compatible. Existing
constructors and builders continue to work unchanged.
Icon.src is validated as required per the spec.
Icon.theme field supports light/dark theme variants.

Includes serialization, deserialization, round-trip, and backward
compatibility tests for all modified records.

Closes #694

Co-authored-by: Sainath Reddy Bobbala <bsnr@amazon.com>

Author: sainathreddyb