Add Notify.Email support for emailing all project members

[AlexCatarino]

Description

Adds engine-side support for sending a Notify.Email notification to all members in the project: passing a null address (Notify.Email(null, ...) / self.notify.email(None, ...)) now means the email is sent to every project member instead of throwing.

Changes

• NotificationEmail: a null address is allowed and means "all members in the project"; empty or whitespace addresses throw ArgumentException instead of silently falling back; addresses are trimmed before validation.
• NotificationEmail.Address is always serialized, as null when unset (explicit NullValueHandling.Include), since the cloud API validates the presence of the Address key.
• NotificationJsonConverter: a missing or JSON-null Address token deserializes to a null address so all-members emails round-trip (a null JToken stringifies to "", which would throw).
• Docs updated on both NotificationManager.Email overloads and the NotificationEmail constructor.

Testing

• Unit tests for the null-address semantics, empty-address rejection, Python notifier.email(None, ...) binding, and JSON round-trip without an address.
• NotificationEmailLiveTests (marked Explicit): enqueues via Notify.Email(null, ...) and sends the dequeued notification through the QuantConnect cloud API (notifications-endpoint, job-user-id and api-access-token config). Verified end-to-end: the API accepts the payload and the email is delivered, including for projects without a live deployment.