Campaign storage schema and `CampaignData` struct

[ayshadogo]

User Story:
As a developer, I want the campaign contract's storage keys and core data struct defined together so that storage access is consistent, typed, and gas-efficient.

Acceptance Criteria:

• DataKey enum: CampaignData, MilestoneData(u32), DonorData(Address), TotalRaised, ContractStatus (plus keys needed by later issues: RaisedPerAsset, Locked, Admin, Frozen)
• CampaignData struct: creator: Address, goal_amount: i128, raised_amount: i128, end_time: u64, status: CampaignStatus, accepted_assets: Vec<AssetInfo>, milestone_count: u32; derives contracttype, stored under DataKey::CampaignData
• Storage access extracted into a dedicated storage module rather than scattered env.storage() calls throughout the contract
• Document, with brief cost reasoning, which keys are temporary vs. persistent storage

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