feat: implement Ed25519 signature validation for price updates

[dominiccreates]

this pr closes #269
This pull request resolves the MITM vulnerability where off-chain endpoints could submit unsigned data payloads to the oracle. It enforces that all ingested payloads must be cryptographically signed by pre-approved validator keys before updates are saved.

Technical Implementation
Storage Scheme Extensions (types.rs):

Added a Validator(BytesN<32>) variant to DataKey to hold authorized validator keys.
Introduced PricePayload struct to represent the details being signed by validators off-chain:
rust

#[contracttype]
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct PricePayload {
pub asset: Symbol,
pub price: i128,
pub decimals: u32,
pub confidence_score: u32,
pub ttl: u64,
}
Core Verification Logic (lib.rs):

Added administration methods: add_validator, remove_validator, and is_validator.
Updated update_price parameter signature to require signature: BytesN<64> and public_key: BytesN<32>.
Verified that public_key is registered via is_validator (rejecting unauthorized updates with Error::Unauthorized).
Reconstructed PricePayload from parameters and verified its off-chain cryptographic signature using env.crypto().ed25519_verify().
Test Suite Adaptation (test.rs):

Added ed25519-dalek to test dependencies to dynamically generate real keypairs and valid Ed25519 signatures inside tests.
Wrote wrappers do_update_price and do_try_update_price to automatically handle serialization and signing in tests.
Updated all 45+ existing test assertions to seamlessly supply valid signature arguments.
Added a dedicated unit test test_ed25519_signature_validation to verify:
Unregistered validator signatures are rejected with Error::Unauthorized.
Authentic signatures from pre-approved validator keys are processed successfully.
Keys successfully removed by the admin are rejected.

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