[Spike] Allow extensions to contribute custom convention for environment-specific operational configuration

**Timeboxed to 2 weeks**; don't spend extra time if done earlier

Different platforms use different ways for configuring operational aspects of application in test/dev/production. LB4 should make it easy to write extensions to support these different platforms.

The goal of this spike is to find out and document what's possible today, identify gaps, propose solutions and create a list of follow-up tasks.

Few examples of operational config:
- configure datasources from environment variables
- use a faster but less-secure hashing algorithm in dev/test (a hashing algorithm is used e.g. to store user passwords)
- apply datasource configuration provided by `VCAP_SERVICES` env variable in CloudFoundry/IBM Cloud - see https://github.com/strongloop/loopback-next/pull/1574

Possibly related: https://github.com/strongloop/loopback-next/pull/983 and https://github.com/strongloop/loopback-next/issues/1396

Start by reviewing the current LoopBack 4 extension and operational-configuration support, then read related pull requests 1574 and 983 and issue 1396. Document what is possible today, identify gaps, propose solutions, and list concrete follow-up tasks; completion is the documented spike outcome within the two-week timebox.

Written by the indexing model from the issue text.