CANTINA-953: Add vip_block_wp_mail filter

[rebeccahum]

Description

This PR introduces a filter vip_block_wp_mail which can be used on the application level in client-mu-plugins to disable email. However, the existing constant VIP_BLOCK_WP_MAIL which is for VIP internal use will take precedence.

Changelog Description

Filter Added: vip_block_wp_mail

New filter vip_block_wp_mail which can be used on the application level in client-mu-plugins to disable email.

Pre-review checklist

Please make sure the items below have been covered before requesting a review:

• This change works and has been tested locally (or has an appropriate fallback).
• This change works and has been tested on a Go sandbox.
• This change has relevant unit tests (if applicable).
• This change uses a rollout method to ease with deployment (if applicable - especially for large scale actions that require writes).
• This change has relevant documentation additions / updates (if applicable).
• I've created a changelog description that aligns with the provided examples.

Pre-deploy checklist

• VIP staff: Ensure any alerts added/updated conform to internal standards (see internal documentation).

Steps to Test

1. Use add_filter( 'vip_block_wp_mail','__return_true ); in client-mu-plugins
2. expect Noop Mailer

[codecov]

Codecov Report

Merging #4975 (3bdd36e) into develop (09fffbe) will increase coverage by 0.33%.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##             develop    #4975      +/-   ##
=============================================
+ Coverage      28.48%   28.81%   +0.33%     
- Complexity      4742     4743       +1     
=============================================
  Files            278      278              
  Lines          20904    20907       +3     
=============================================
+ Hits            5954     6025      +71     
+ Misses         14950    14882      -68     

Files	Coverage Δ
vip-mail.php	75.26% <100.00%> (+4.15%)	⬆️

... and 14 files with indirect coverage changes

[mahbubme]

There is a typo in the documentation here, can someone update this? It should be '__return_true' instead of '__return_true

[rebeccahum]

Thanks @mahbubme ! This has been fixed now. cc @yolih