Subject and content in mail send API should not be required

[rhumie]

Issue Summary

Subject and Content are defined as required according to the following, but they should not be required if you specify the template ID.
https://github.com/sendgrid/sendgrid-oai/blob/main/oai.yaml#L321-L322

[JenniferMah]

This issue has been added to our internal backlog to be prioritized. Pull requests and +1s on the issue summary will help it move up the backlog.

[riceyrice]

One thing I have noticed is that using a dynamic template is the API only sends the content types you provide in the API call, although the content value itself is replaced with template output, so the content type is required, just not the value.

[jbellos]

Ran into the same issue - I was really hoping to use this oai spec to build a mock server with prism/stoplight but this issue causes false assertions with our tests.