API Allow array style email addresses

[emteknetnz]

Issue #10618

Background - symbiote/silverstripe-queuedjobs#395

Array style email addresses - https://docs.silverstripe.org/en/4/developer_guides/email/#administrator-emails

[GuySartorelli]

Should we validate here that array_keys($adminEmail)[0] is in fact a string (or even go so far as to validate it's a valid RFC email address?

I know the second part of that goes beyond the original purpose of this PR so I'm okay if we say we want to do that but make a card to handle it later - but the former part of that seems like it falls in line with this PR's purpose if we want to do it.

[emteknetnz]

Validation happens within symfony so we don't need to do this here, nor should really should we since we're leveraging symfony to do the heavy lifting

e.g. the following config

SilverStripe\Control\Email\Email:
  admin_email:
    aa: 'Admin-email'

and the following script

$email = new Email();
$email->to('[email protected]')->subject('test')->setBody('abc')->send();

Will result in
[Emergency] Uncaught Symfony\Component\Mime\Exception\RfcComplianceException: Email "aa" does not comply with addr-spec of RFC 2822.

[GuySartorelli]

What about

SilverStripe\Control\Email\Email:
  admin_email:
    - '[email protected]'

nvm of course that will have the same complaint, Email "0" does not comply etc