Allow passing more than one context to an extension

Currently, an extension process is invoked with environment variables containing credentials and configurations for the current context. @VimCommando highlighed a use case where passing multiple contexts to an extension would be valuable: piping data from one Elasticsearch cluster to another.

Some discovery and experimentation is still needed to identify the most secure and ergonomic way to implement this feature.

Start by tracing how an extension process currently receives environment variables for the current context. Explore secure and ergonomic ways to pass multiple contexts, using the Elasticsearch-to-Elasticsearch piping use case as the acceptance scenario. Done means the approach is defined clearly enough to implement and its security implications are addressed.

Written by the indexing model from the issue text.