feat(signatures): add email-related signatures to type enumeration

## Use case

- `SignatureTypes.SIG_TYPE_MAIL_SOURCE_EMAIL`
- `SignatureTypes.SIG_TYPE_MAIL_PLAYER_EMAIL`
- `SignatureTypes.SIG_TYPE_MAIL_URL_HASH`
- `SignatureTypes.SIG_TYPE_MAIL_ATTACHMENT_HASH`

## Current workaround

None

## Proposed solution

Add elements to `pyoaev.signatures.types.SignatureTypes` with values following the usual pattern.
