feat(program): Implement course and rate limit functionality

[olasunkanmi-SE]

• Introduce a new 'Course' struct to represent courses, including course name, NFT mint, capacity, and tuition fee
• Update 'School' struct to remove school type enum and replace it with a string
• Implement rate limiting on minting operations with a new 'RateLimit' struct
• Introduce a new 'SchoolTypeTrait' to verify if a school type is valid
• Update 'NFTCreator' trait to enforce rate limits and create NFT metadata accounts
• Introduce a new 'NFTAccounts' struct to represent NFT accounts, including mint, associated token account, and metadata account