Added MailSlurp adapter

[unickorn]

What does this PR do?

Adds the MailSlurp email adapter. The exact limit for getMaxMessagerPerRequest() is not documented, but I emailed the MailSlurp support team for more info and left a TODO there for now.

Test Plan

I've created a free MailSlurp personal plan, added a MailSlurpTest.php file and tested it successfully.

Related PRs and Issues

appwrite/appwrite#6390

[unickorn]

MailSlurp support team has not responded, so I'm leaving the TODO there as is. Unfortunately can't find documentation about it still.

[gewenyu99]

Hey,

Due to time constraints, I'm going to mark this PR hacktoberfest-accepted for now so you get DO's Hacktoberfest rewards. We'll continue to work with you on this issue for review and merge.

When it is merged, we'll contact you for Appwrite-specific Hacktoberfest swag.

Thanks for helping us improve Appwrite!

[gewenyu99]

Hey there! There were a lot of big PRs during this Hacktoberfest, and we wanted to give everyone ample time to collaborate with our engineering team. If you were able to merge your PRs during October, amazing. If it’s still not merged, don’t worry about it either. Either way, we’ve got your Hacktoberfest swag minted and ready to ship.

Please comment with your Discord username here so we can contact you about your shipping information to deliver your Hacktoberfest swag.

[gewenyu99]

Will reach out soon. Putting together a form for y'all!