add "contains" field to custom cache key

[anthocf]

The Cloudflare CDN team has introduced changes to the Cache Rules API so customers can shard cache based on headers they include in the cache_key.custom_key.header.contains field of their cache rules. I am working on Terraform support and need this change to go in first.

Description

This change adds support for a new field. It is required for the Terraform provider support to work.

Has your change been tested?

Unit tests in this PR.

Screenshots (if appropriate):

N/a

Types of changes

What sort of change does your code introduce/modify?

• 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)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• This change is using publicly documented in cloudflare/api-schemas
  and relies on stable APIs.

[github-actions]

changelog detected ✅

[github-actions]

This functionality has been released in v0.103.0.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!