fix: handle malformed deposit requests

[matthias-wright]

Builds on #202 (which builds on #167 and #192).

This addresses #201.

Changes:

• Introduces an enum ParsedExecutionRequest with variants Valid(ExecutionRequest) and MalformedDeposit(MalformedDepositRequest). ExecutionRequest is the already existing execution request type, and MalformedDepositRequest is a new type for a deposit request with invalid keys.

pub enum ParsedExecutionRequest {
    Valid(ExecutionRequest),
    MalformedDeposit(MalformedDepositRequest),
}

• Arriving execution requests are parsed into ParsedExecutionRequest first, and then processed. Malformed deposit requests will be rejected and a withdrawal will be initiated similarly to deposit requests that contain invalid signatures.
• Unit tests are added to ensure that a malformed deposit request cannot drop valid same-entry deposit requests.

[evonide]

LGTM. This PR parses grouped deposit entries per 288-byte deposit chunk so a malformed deposit is isolated/refunded while valid same-entry deposits still reach normal processing.