feat(contracts): add configurable campaign participation fee in custom tokens (#1371)

[frammawiliansyah]

Description

Resolves #1371

Enables campaign creators to configure participation fees using custom Soroban token addresses rather than strictly native tokens, extending reward mechanics and treasury flexibility.

Key Changes

• contracts/campaign/src/lib.rs: Added token identifier and fee parameter configuration to campaign initialization and participation structures.
• contracts/campaign/src/test.rs: Added comprehensive Soroban unit tests verifying fee deductions, custom token transfers, and zero-fee fallback handling.

Verification

• Ran contract unit tests via cargo test passing cleanly.
• Tested token transfer allowance checks and edge-case fee validation.