Sendgrid: Allow to specify the content type

[firefart]

Description

Previously the Content-Type was hardcoded to text/html. This enables the use of text/plain like in the mail service.
Also the sendgrid library now has a SendWithContext method which makes the code simpler.

Motivation and Context

It's now possible to send plain text emails

How Has This Been Tested?

Sending an email through sendgrid. Most of the code was copied over from the mail service where the same functionality already exists.

Screenshots / Output (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 22 lines in your changes missing coverage. Please review.

Project coverage is 29.62%. Comparing base (8efdc15) to head (a50edb4).
Report is 38 commits behind head on main.

Files with missing lines	Patch %	Lines
service/sendgrid/sendgrid.go	0.00%	22 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #871       +/-   ##
===========================================
- Coverage   74.10%   29.62%   -44.48%     
===========================================
  Files          44       61       +17     
  Lines        1572     2646     +1074     
===========================================
- Hits         1165      784      -381     
- Misses        307     1741     +1434     
- Partials      100      121       +21     

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[nikoksr]

LGTM; thank you for your contribution, @firefart.