Loosen scriban validation

[tevoinea]

Summary of the Pull Request

What is this about?

When strict template validation is enabled, we enforce that keys used in the template must exist in TemplateRenderContext's dictionaries in order to pass validation. However, this causes an issue with our validation because we can't know every possible key that could go in the dictionary.

This PR loosens the rule that the keys used in the template must exist in the render context.

Info on Pull Request

What does this include?

• Loosen the rule
• Test to validate the change -- in the test, task.tags.project does not exist in the template render context, and the validation is still successful

[codecov-commenter]

Codecov Report

Merging #2963 (c718a4e) into main (34b513e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2963   +/-   ##
=======================================
  Coverage   29.07%   29.07%           
=======================================
  Files         304      304           
  Lines       36335    36335           
=======================================
  Hits        10566    10566           
  Misses      25769    25769           

Impacted Files	Coverage Δ
...vice/onefuzzlib/notifications/NotificationsBase.cs	69.00% <100.00%> (ø)

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