feat: more mail sending backends and plugability

[matmair]

This introduces models to track mail delivery and receiving - and mail processing.

Todo:

• Mail receiving
• Passing received mails to plugins
• Passing outgoing mails to plugins
• Admin UI to view mails
• doc improvements
• maybe: send test mail

Closes #7950
Closes #7951

I have validated sending against:

• SMTP with cpanel
• SMTP with Postfix
• Sending / receiving with Mailgun
• NOT - avoid them! - Sending / receiving with Sendgrid
• Sending with Mailjet

I have validated receiving againsz:

• Mailgun
• Mailjet

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[codecov]

Codecov Report

Attention: Patch coverage is 97.72152% with 9 lines in your changes missing coverage. Please review.

Project coverage is 85.57%. Comparing base (6b261e1) to head (d15fa63).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
src/backend/InvenTree/InvenTree/helpers_email.py	40.00%	6 Missing ⚠️
src/backend/InvenTree/InvenTree/middleware.py	50.00%	2 Missing ⚠️
src/backend/InvenTree/InvenTree/settings.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9608      +/-   ##
==========================================
- Coverage   86.34%   85.57%   -0.77%     
==========================================
  Files        1232     1238       +6     
  Lines       54154    55844    +1690     
  Branches     2248     2259      +11     
==========================================
+ Hits        46758    47789    +1031     
- Misses       6827     7483     +656     
- Partials      569      572       +3     

Flag	Coverage Δ
backend	88.36% <97.70%> (+0.10%)	⬆️

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