This commit adds an admin API route and handler for
requesting status information about a KMS key.
Therefore, the client specifies the KMS key ID (when
empty / not set the server takes the currently configured
default key-ID) and the server tries to perform a dummy encryption,
re-wrap and decryption operation. If all three succeed we know that
the server can access the KMS and has permissions to generate, re-wrap
and decrypt data keys (policy is set correctly).
