Support "contentguard" configuration on "setup.products.bot_management"

[harmony7]

Change summary

This PR adds support to the ContentGuard field under "bot_management" under setup.products.

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• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

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Changes to Core Features:

• Have you written new tests for your core changes, as applicable?
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User Impact

Nothing should be broken.

contentguard is required and must be either the string "on" or "off" - https://www.fastly.com/documentation/reference/api/products/bot_management/#field_contentguard

Are there any considerations that need to be addressed for release?

[rcaril]

Looks good, thanks for adding this! However, I believe the changelog entry needs to be added still?

[harmony7]

I've added the changelog.