Add support for a custom cache policy

[dmattia]

what

• AWS has a relatively new CloudFront feature named Cache Policies: https://aws.amazon.com/blogs/networking-and-content-delivery/amazon-cloudfront-announces-cache-and-origin-request-policies/
• This adds support for the default cache to set an explicit cache policy
• This also sets restrictions on not allowing some other params to be set if the cache policy is specified, as is consistent with the AWS Console's wizard when creating a distribution.

This module could also create a cache policy itself if desired, I would gladly make that change.

Note: I support a module for creating cache policies: https://registry.terraform.io/modules/transcend-io/cache-policy/aws/latest

why

• Cache Policies are the easiest (only?) way to add brotli compression in CloudFront

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[justnom]

@dmattia

Should we also add the cache_policy_id into the ordered_cache map?

Similar to this commit: cloudposse/terraform-aws-cloudfront-cdn@12cbfb4

[dmattia]

We definitely could do that as well. Would you like me to make that change?

[justnom]

We definitely could do that as well. Would you like me to make that change?

Sure, or I can do it in a separate PR after this is merged. I can't seem to suggest changes to the PR from outside the scope shown in "Files changed".

[dmattia]

@justnom Awesome, let's do the ordered caches in a separate PR then 👍

[mergify]

This pull request is now in conflict. Could you fix it @dmattia? 🙏

[mergify]

This pull request is now in conflict. Could you fix it @dmattia? 🙏

[mergify]

This pull request is now in conflict. Could you fix it @dmattia? 🙏

[Gowiem]

@dmattia apologies for giving you conflicts yet again. This looks like a straight forward change... want to update once last time and then ping me and we'll get this merged for you?

[Gowiem]

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