Adding Mailpit Support

[SirLouen]

Adding support for a Mailpit container on demand with the env variable: LOCAL_MAIL

Patch Testing instructions:

1. Change LOCAL_MAIL=false to LOCAL_MAIL=true in your .env file
2. Run your docker environment as usual npm run env:start
3. You test if your mailhog instance is running with either:
   A) Go to http://localhost:8025/
   B) You can run in your wordpress-develop container

docker exec -it  wordpress-develop-php-1 openssl s_client -connect mailhog:1025

And it might result in something like:

CONNECTED(00000003)
135865530688832:error:1408F10B:SSL routines:ssl3_get_record:wrong version number:../ssl/record/ssl3_record.c:331:
---
no peer certificate available
---
No client certificate CA names sent
---
SSL handshake has read 5 bytes and written 283 bytes
Verification: OK
---
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
Early data was not sent
Verify return code: 0 (ok)

C) You can do further tests, like sending an email with a plugin that uses the local mailhog hostname
For example this one: https://github.com/SirLouen/amazon-s3-email-testing/archive/refs/tags/1.0.0.zip

Trac ticket: https://core.trac.wordpress.org/ticket/63135

---

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Props sirlouen, mindctrl.

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[mindctrl]

@SirLouen is there anything left to do here?

[SirLouen]

@mindctrl yeah this is still of phase 1.
But I'm using this mainly to drag it as a patch and test reports that need something relating with Mail component.
I don't think it will get through as-is right now.

[mindctrl]

@SirLouen could you add a list of things left to do in the opening description? We have a 6.9 milestone and I'd love to help get it in the proper state for a committer to review/merge. I've needed local mail testing multiple times recently. WCUS Contributor Day is coming up and I know a few people willing to help.

[SirLouen]

@mindctrl I think it's time to de-milestone this. I was confident that this could be a great addition to wordpress-develop, but for now it's not adding value in the line that is expected (according to one dev-chat some months ago)

My idea is to look to add extra value to this, but this is not going to happen anytime soon, and I seriously doubt it will make it on time for 6.9. This PR by itself is useful for someone like me that uses this PR like every other day, but useless for most of the people that seldom test anything—mail. Hope to see it merged somewhere in the future, but much work is needed to consider it a potential merge (mainly creating integration tests in PHPUnit that make use of this service).

I've updated the milestone so we don't need to rush it anymore.