[Fleet] Create additional API Keys for inputs

[simitt]

Some inputs require access to Elasticsearch with additional privileges compared to the ones provided for the output.
An example for such a requirement is the new tail based sampling feature in APM Server, requiring read access to a specific data stream in Elasticsearch.

For such requirements an API Key should be generated and passed down to the Elastic Agent, which in turn will need to split up the configuration into inputs and forward the keys to the right inputs.

[elasticmachine]

Pinging @elastic/fleet (Feature:Fleet)

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[ph]

@simitt Looking at elastic/fleet-server#101 we won't need to create additional api key for input but actually we will generate finer grained permission on the api keys.