Add `confluent_certificate_pool` data source and resource

[sgagniere]

Testing: https://docs.google.com/document/d/1aFrUi1vzWtBkIUwagxs8BQYZoyg3wv3SUjBz6kD3TBY/edit

[confluent-cla-assistant[bot]]

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[cla-assistant[bot]]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
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