[CONTRACT] Implement Asset Tokenization in Asset Contract

[truthixify]

Description:
Extend the asset.rs contract to support asset tokenization for Pro/Enterprise plans, storing a Stellar token ID.

Tasks:

• Add a tokenize_asset(asset_id: BytesN<32>, token_id: BytesN<32>) function to asset.rs.
• Restrict tokenization to the contract admin (set in initialize).
• Update the Asset struct to store stellar_token_id.
• Use errors.rs for errors (e.g., AssetNotFound, Unauthorized).
• Write tests for tokenization and access control.
• Document the tokenization process.

Acceptance Criteria:

• Only the admin can tokenize assets.
• Tokenization updates stellar_token_id in the Asset struct.
• Tests confirm tokenization works and unauthorized access is blocked.
• Contract compiles without errors.

[ISTIFANUS-N]

Can I work on this?

[truthixify]

It’s yours @ISTIFANUS-N