[WPB-9859] Support OAUTH SMTP authentication

[stefanwire]

https://wearezeta.atlassian.net/browse/WPB-9859

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines

[fisx]

looks good so far i think (assuming you're adding what we just discussed later today).

i agree that we should also write a test in /integration. as an easy way to commit the test to the PR without breaking things when the credentials evaporate, we can deactivate the test by default, printing an info "if you want this test to run, you need to provide credentials in the source code and remove the pending").

[fisx]

Suggested change

	parseJSON = withObject "smtpCredentials" $ \v ->
	parseJSON = withObject "EmailSMTPCredentials" $ \v ->

i like the habit of using the haskell type as description verbatim.

[fisx]

this is fine, but you could also:

Suggested change

	v .:? "smtpAuth" .!= ("basic" :: String) >>= \case
	"xauth2" ->
	EmailSMTPXAUTH2
	<$> v .: "smtpUsername"
	<*> v .: "smtpXAUTH2Token"
	_ ->
	EmailSMTPBasicAuth
	<$> v .: "smtpUsername"
	<*> v .: "smtpPassword"
	v .:? "smtpAuth" >>= \case
	Just "xauth2" ->
	EmailSMTPXAUTH2
	<$> v .: "smtpUsername"
	<*> v .: "smtpXAUTH2Token"
	_ -> -- this covers the `Just "basicAuth"` case and the fallback for everything else.
	EmailSMTPBasicAuth
	<$> v .: "smtpUsername"
	<*> v .: "smtpPassword"

(not tested)

(or should there be a type error for everything but Nothing in the fallback rule?)