Support opaque `api_key`

[robbavey]

The Elasticsearch output accepts an api_key parameter, which is expect to be in the name:key format. This value is base64-encoded into an opaque value before being used in an Authorization: ApiKey #{opaque} HTTP header.

To reduce friction, we should accept the opaque value for api_key anywhere that we accept the name:key format api_key. We can do so by assuming that the value is opaque unless it includes a colon (:) character.