Support context variables in the entire Elastic Agent configuration

[blakerouse]

Describe the enhancement:

At the moment variables from composable providers are supported in the inputs.* only. Support for this variables outside of the inputs.* is required to allow other areas, like outputs or fleet to use values.

Issue with this change is at the moment go-ucfg provides the variable replacement outside of inputs.* and that supports the a very similar syntax that will conflict. To solve this issue we can add a fallback where if no prefix of the provider is provided for a variable then it will default to the env provider which is all the go-ucfg supports at the moment. That allows the replacement of go-ucfg outside of inputs.*. Removing the need to support any variable replacement from go-ucfg.

Describe a specific use case for the enhancement or feature:

This is needed to allow provides to provide variables outside of inputs, explicitly because provider variables can change through out the live of the Elastic Agent.

What is the definition of done?

When all context providers work outside of inputs. and go-ucfg variable replacement can be removed.

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)