Allow getting/setting publicPreview flag through API for Emails

[kuzmany]

Q	A
Bug fix? (use the a.b branch)	[ ]
New feature/enhancement? (use the a.x branch)	[X]
Deprecations?	[ ]
BC breaks? (use the c.x branch)	[ ]
Automated tests included?	[ ]
Related user documentation PR URL	mautic/mautic-documentation#...
Related developer documentation PR URL	mautic/developer-documentation#...
Issue(s) addressed	Fixes #...

Description:

This PR allow to get and set email flag publicPreview through API.

Steps to test this PR:

1. Open this PR on Gitpod or pull down for testing locally (see docs on testing PRs here)
2. Send API request for an email:

GET {{host}}/api/emails/{email_id}
Accept: application/json
Authorization: Bearer {{auth_token}}

the response will containt

"publicPreview": true,

update existing email by putting

"publicPreview": true,

into payload body

the post value can be either:
true: true,"T","Y"
false: false,"F","N"

and verify it has been updated in UI

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.08%. Comparing base (693eaf3) to head (ed99162).

Additional details and impacted files

@@            Coverage Diff            @@
##                5.x   #13609   +/-   ##
=========================================
  Coverage     61.08%   61.08%           
- Complexity    33957    33960    +3     
=========================================
  Files          2238     2238           
  Lines        101510   101515    +5     
=========================================
+ Hits          62003    62007    +4     
- Misses        39507    39508    +1     

Files	Coverage Δ
.../EmailBundle/Controller/Api/EmailApiController.php	89.39% <100.00%> (+0.86%)	⬆️
app/bundles/EmailBundle/Entity/Email.php	90.29% <ø> (ø)

... and 1 file with indirect coverage changes

[escopecz]

No issue from the code perspective 👍