use instance_id_ variables For Multi Select Endpoints

[shubham1g5]

Technical Summary

This is a modification on top of the multi-select session endpoints support. Instead of substituting the instance arguments directly, use a prefix instance_id_argname to signify that we want to substitue the argument with corresponding instance guid.

Safety Assurance

Safety story

Automated test coverage

QA Plan

Migrations

• The migrations in this code can be safely applied first independently of the code.

Special deploy instructions

• This PR can be deployed after merge with no further considerations.

Rollback instructions

• This PR can be reverted after deploy with no further considerations.

Review

• The set of people pinged as reviewers is appropriate for the level of risk of the change.

cross-request: dimagi/commcare-core#1327

[shubham1g5]

@snopoke This needs cleanup but I kind of just want to get a OK on the approach here before making further changes here and on HQ. This concern came up whille working on inline case search changes which might need us to retain both the original argument value along with the substituted instance argument and I would prefer to keep the actual argument name as it is.

I confirmed with delivery that this is not deployed yet in any of their apps so should not need backward compatibility.

[snopoke]

I don't really like this approach because it depends on some internal naming convention in the engine.

I made this comment on the spec as well but could not use the instance to get the list of case IDs in the query:

<data key="case_id" nodeset="instance('selected_cases')/results/value" ref="."/>