[Contracts] Add advanced participant verification

[Kaylahray]

Description

The contract README notes advanced participant verification as a future enhancement, but giveaway entry currently only checks auth, timing, and duplicate participation.

User Story

As a giveaway creator,
I want to restrict participants by extra conditions,
so that I can run allowlisted or reputation-gated campaigns.

Requirements and Context

• Files: contracts/geev-core/src/giveaway.rs, contracts/geev-core/src/types.rs, contracts/geev-core/src/test.rs
• Add support for entry allowlists or minimum reputation requirements
• Enforce the verification rule before a participant is accepted
• Keep the normal entry path unchanged when no extra checks are configured

Suggested Implementation

// PARTICIPANT VERIFICATION
// 1. Add optional verification rules to giveaway state.
// 2. Check allowlist or reputation before accepting an entry.
// 3. Reject unauthorized entries with a clear error.
// 4. Preserve existing duplicate-entry protections.

Acceptance Criteria

• Allowlisted or reputation-gated giveaways reject unqualified participants.
• Open giveaways still accept normal entries.
• Verification failures are surfaced clearly in tests.

Submission Guidelines

• Branch: feat/participant-verification
• Depends on: giveaway entry flow
• PR: feat(contracts): add participant verification rules

[Goldy042]

@Goldy042 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello, I went through the issue details and relevant files and would like to take this up. I’ll keep the implementation focused, well-tested, and easy to review. Please assign it to me if available

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Goldy042 to this issue.