Follow environment config naming convention and reuse existing variables from rollups-node

[endersonmaia]

Context

Just like cartesi/rollups-graphql#30, we should follow a naming convention between other optional/auxiliary rollups-node services, since they're probably going to run alongside each other and potentially share the same environment.

Proposal

Environment variables that controls cartesi-rollups-espresso-reader should be prefixed with CARTESI_ESPRESSO_.

Environments variables that points to the same resources used by the rollups-node should be reused, like CARTESI_DATABASE_CONNECTION (#34)

[endersonmaia]

I'm assuming those also are used by the rollups-espresso-reader:

• ESPRESSO_SERVICE_ENDPOINT -> CARTESI_ESPRESSO_SERVICE_ENDPOINT
• ESPRESSO_BASE_URL - > CARTESI_ESPRESSO_BASE_URL