[SMART CONTRACT] Implement Soulbound Token (SBT) Identity

[EDOHWARES]

Description
Develop a non-transferable token contract used for identity verification or achievement tracking on Soroban.

Requirements and context

• Prevent all transfer calls.
• Authority-driven minting and burning.
• Metadata storage for identity attributes.

Suggested execution

• Work in contracts/identity_sbt/.
• Follow SEP-0041 and override transfer logic.

Example commit message
feat: implement soulbound token sbt for identity verification