ZEVA2 - email service module

**Describe the task**
Create a module that exports functions that allow developers to send emails using CHES. Please see the `send_email.py` file in `backend/api/services` of `zeva1` for inspiration.

**Purpose**
This will allow us to send emails in the bullmq service (and elsewhere) when certain actions are performed in the system (e.g. a credit transfer is initiated, a new zev is submitted, etc.)

**Acceptance Criteria**
- [ ] Define the needed environment variables (e.g. CHES clientId, clientSecret, etc.) in docker-compose
- [ ] Implement a getToken() function that gets a CHES access token (can use zeva1's CHES credentials for now)
- [ ] Implement a sendEmail() function that gets a CHES access token, and sends an email using the CHES API + the token