Cannot use disk buffers when passing through an API key from datadog_agent -> datadog_logs

[jszwedko]

Related: #7065

In #7394 we added support for storing the API key the Datadog Agent sent events with in the datadog_agent source. This enables re-using the same API key in the datadog_logs source to let Vector be a sort of proxy for the Datadog Agent.

However, when disk buffers are used, the metadata is not persisted and so the API key is lost and thus breaks this proxy behavior.

One consideration is that this will result in sensitive data being written to disk. We could consider allowing some sort of encryption of this field.