Friendly environment variables setup

[gusinacio]

Problem statement
Currently, each service has its own tag (INDEXER_SERVICE and TAP_AGENT), but at the same time, they are using shared variables. This makes a lot harder to work with especially in some setups like docker-compose where yaml anchors would be a perfect fit to share config.

Expectation proposal
Create a backward-compatible way to provide a single environment variable that serves both services.
The new tag should be something like INDEXER_, suggestions are welcome.