Add token metadata URI extension (off-chain asset details)

[Prasiejames]

Problem

The RWA token stores only name, symbol, and decimals on-chain. Real-world assets need additional metadata: asset documentation, legal disclaimers, valuation reports, prospectus links, etc.

Expected Behavior

The token should support a metadata URI (like ERC-721/SEP-41 metadata extension) pointing to a JSON document with full asset details.

Proposed Solution

Add to rwa-token:

pub fn set_metadata_uri(env: Env, uri: String) { /* admin only */ }
pub fn get_metadata_uri(env: Env) -> Option<String> { /* ... */ }
pub fn remove_metadata_uri(env: Env) { /* admin only */ }

The URI should point to a JSON document following the SEP-41 metadata schema (or a custom schema documented in the repo).

Affected File

• contracts/rwa-token/src/lib.rs