Implement token vesting contract module

[Xaxxoo]

Summary

• Add a new Soroban token vesting workspace contract for linear schedules with cliffs.
• Support single and batch beneficiary funding, claim and batch claim flows, early withdrawal penalties, admin modification, revocation, and schedule top-ups.
• Add token-backed unit tests and document the new module in the README.

Tests

• cargo fmt --all --check
• cargo test -p token-vesting --lib -j 1
• cargo build --all --locked -j 1
• cargo clippy --all --all-targets --all-features -j 1 -- -D warnings

Closes #275