[Meta][Shipper] Support the Elastic Agent Data Shipper

[cmacknz]

The Elastic agent must be capable of starting the Elastic agent data shipper and connecting inputs to it. The data shipper implementation is tracked under elastic/elastic-agent-shipper#3

To support the phase 1 data shipper the agent must complete at least the following tasks:

• [Meta][Feature] Integrate the shipper into the elastic agent packaging step #216
• Define a feature flag for enabling the shipper in the agent policy, defaulting to false. #217
• [Meta][Feature] Start the shipper process(es) and configure inputs to write to them when the shipper feature flag is enabled. #219
  • Always provide the Elasticsearch output unit configuration to Heartbeat when using the shipper #1476
  • Provide the relevant subset of the agent policy to the shipper as its configuration #617
  • [Meta][Feature] The agent policy's input processor configurations must be provided to the shipper. #220
  • The number of shippers to start will be determined based on the number of outputs requested and their configuration. There is a one to one mapping between outputs and shipper processes initially.

[cmacknz]

I created #617 to discuss the configuration the agent will need to provide to each shipper instance it starts.

[blakerouse]

We need to discuss how we want to implement a solution for #1476.