This commit adds a `context.Context` to the
the KMS `{Stat, CreateKey, GenerateKey}` API
calls.
The context will be used to terminate external calls
as soon as the client requests gets canceled.
A follow-up PR will add a `context.Context` to
the remaining `DecryptKey` API call.
Signed-off-by: Andreas Auernhammer <hi@aead.dev>