feat: add second example template: token-allowlist

[maztah1]

closes #115

This PR resolves issue #115 by adding a second built-in example template: token-allowlist in templates/examples/token-allowlist/.

Key Changes

1. Added Example Template token-allowlist:
   • Cargo.toml: Package manifest using the standard Soroban configuration and {{PROJECT_NAME}} placeholder.
   • README.md: Explaining the template's features, build, test, deploy steps, and CLI invocation examples.
   • src/lib.rs: Full Soroban smart contract for a token allowlist. It allows an admin to manage approved addresses, query status, and transfer/update admin authority, incorporating {{PROJECT_NAME_PASCAL}} placeholder. Includes robust unit tests.
2. Updated Documentation:
   • Updated templates/README.md to document the new token-allowlist template.
3. Updated Integration Tests:
   • Updated tests/template_marketplace_test.rs to verify the structure, files, and placeholders for both simple-counter and token-allowlist example templates.

Verification Plan

Automated Verification

• Ran structural and placeholder verification successfully to ensure all template files contain valid standard layouts and expected placeholders:
  • {{PROJECT_NAME}} in Cargo.toml
  • {{PROJECT_NAME_PASCAL}} in src/lib.rs

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