Add support for env vars from v0.3.1

[JoshMock]

Prior to 0.4.0, when the implementation switched from Node.js to Rust, there were a few arguments/env vars that were supported that are no longer present:

• ES_CA_CERT
• ES_URL should support several URLs (see also Support multiple hosts / es urls #49)
• ES_VERSION
• ES_PATH_PREFIX

Docs for how they were to be used are available here.

When adding ES_VERSION back, we would additionally be able to close #118 if we add 7 to the list of valid choices.

[swallez]

• ES_PATH_PREFIX is not needed, as it's part of ES_URL. We should allow for several URLs though for cases where there's not a load-balancer in front of the ES nodes
• ES_VERSION is no more needed either. The only version-dependent tool is ES|QL that was introduced as tech preview in 8.11. We may want to hide this tool in pre-8.11 versions, and this requires more than 8 or 7 as values. A better UX would be to implement it by asking ES for its version at initialization time.

I edited the description to reflect the above.