Swappable email config

[timkelty]

This changes the schema of the email node in project config, so that you can have multiple configured transports represented, and change the active one.

email.transportType and email.transportSettings are replaced with email.transport and email.transports:

email:
  fromEmail: [email protected]
  fromName: 'Pixel & Tonic'
  replyToEmail: null
  template: ''
  transport: $EMAIL_TRANSPORT
  transports:
    postmark:
      type: craftcms\postmark\Adapter
      settings:
            token: $POSTMARK_TOKEN
    mailhog:
      type: craft\mail\transportadapters\Smtp
      settings: 
            encryptionMethod: none
            host: localhost
            password: ''
            port: '1025'
            timeout: '10'
            useAuthentication: '0'
            username: ''

[timkelty]

Moving to draft to update/re-evaluate