Add support for email service providers APIs as an alternative to SMTP

[lukevella]

Discussed in #639

Internally we already use the Amazon SES API for rallly.co, however this is not documented publicly yet. That said, it serves as a guide for implementing support for other providers.

A good starting point is send-email.tsx which is where we decide what transport to use based on the env var EMAIL_PROVIDER.

Initially we can support the following service providers but more can be added if requested:

• [x] Amazon SES
• [ ] Mailgun

[cedricwalter]

will be fixed in https://github.com/lukevella/rallly/pull/653