Migrate EventManger away from EventUser model

[NisanthanNanthakumar]

Objective:

Relevant method: _get_event_user_impl

Continue creating the EventUser model record, but we will want to make the Snuba query and return the EventUser dataclass from the results of the Snuba query.

We will continue creating EventUser records until we are confident the Snuba queries are a sustainable and error-free replacement.

[NisanthanNanthakumar]

This is blocked until we want to remove the EventUser model. This function creates the EventUser record if it doesnt exist.

[NisanthanNanthakumar]

We can use the user_data values to populate the values in the EventUser dataclass