Implement Direct SMTP Server Option for Alert Creation

[mastercactapus]

What problem would you like to solve? Please describe: Currently, email-based alert creation in GoAlert is only supported via Mailgun webhook. This feature request aims at providing a cost-effective alternative by allowing GoAlert to work as a basic SMTP server that only receives emails.

Describe the solution you'd like: I propose an SMTP server configuration for GoAlert that enables receiving of mail. This will require admins to configure an MX record for their GoAlert domain. This would function as any other API endpoint, but with a separate port and unique syntax/protocol.

This solution would also mean that we have multiple potential providers of a single service (Email integration keys) for the first time. Multiple providers would need to be managed, either by only allowing one to be enabled at a time, setting a preferred provider, or requiring domains to be identical.

Describe alternatives you've considered: The current alternative is continuing to use Mailgun webhooks for all email-based alerts. This option carries potential costs and fewer customization options.

Additional context: Additional considerations include ensuring that all documentation is updated accordingly, and that the "preferred" provider's domain is used in any in-app links. Potential forward compatibility issues might arise, so it may be beneficial to support both methods of alert creation for migrating from one to the other.

[mastercactapus]

I think this feature can be enabled via CLI flags/env vars.

Flag	Type	Description
--smtp-listen-tls	string	Listen address:port for the SMTP server with TLS.
--smtp-tls-cert-data	string	Specifies a PEM-encoded certificate for the SMTP server. Has no effect if neither --smtp-listen-tls nor --smtp-listen are set.
--smtp-tls-cert-file	string	Specifies a path to a PEM-encoded certificate for the SMTP server. Has no effect if neither --smtp-listen-tls nor --smtp-listen are set.
--smtp-tls-key-data	string	Specifies a PEM-encoded private key for the SMTP server. Has no effect if neither --smtp-listen-tls nor --smtp-listen are set.
--smtp-tls-key-file	string	Specifies a path to a PEM-encoded private key file for the SMTP server. Has no effect if neither --smtp-listen-tls nor --smtp-listen are set.
--smtp-listen	string	Listen address:port for the SMTP server without TLS (though STARTTLS will be enabled if cert/key is specified).
--email-integration-domain	string	This flag is required to set the domain used for email integration keys when --smtp-listen or --smtp-listen-tls are set.
--smtp-allowed-domains	string	Comma-separated list of additional destination domains allowed for SMTP.

[mastercactapus]

Some notes on implementation:

• Flags are defined in app/cmd.go at the bottom and get parsed into app/config.go.
• For a package name to house this code maybe something like smtpsrv or smtpint just so it doesn't clash with the smtp package(s) being used elsewhere.
• We will want to handle address parsing (e.g., things like the dedup value) the same as in mailgun/mailgun.go
• We should validate the RCPT TO address right away as an integration key.
• At first glance, https://github.com/emersion/go-smtp looks to support the features we need
• We'll also want to add a section in the Getting Started Guide near Mailgun for the new SMTP functionality, including setting a proper MX record