Feature Request: Multiple key support

We have many kubernetes clusters. For lots of secrets, we encrypt them per-cluster, as they're unique to that cluster.

For some secrets, we would like to encrypt them once, and have all clusters be able to decrypt those secrets. Examples might be, the newrelic or pagerduty API keys.
Each cluster would have the dynamically generated key that's unique to it, and we would like to use that key to encrypt the global decryption key.

This could be modelled by allowing SealedSecrets to optionally reference a different decryption key, potentially?

Start by reviewing how SealedSecrets currently model cluster-specific encryption keys and decryption. Define the proposed relationship between a shared global decryption key and each cluster key, then identify the required behavior and acceptance criteria before locating implementation and tests.

Written by the indexing model from the issue text.