fix(embedded): sankey charts

[betodealmeida]

SUMMARY

A saved Sankey chart query context has the source and target columns in the groupby field:

{
    "queries": [
        {
            ...
            "columns": [],
            "metrics": ["count"],
            "groupby": ["bot_id", "channel_id"],  # here
         },
    ],
}

But the query context from the same chart, when embedded, looks like this:

{
    "queries": [
        {
            ...
            "columns": ["bot_id", "channel_id"],
            "metrics": ["count"],
            # no "groupby" key
         },
    ],
}

This PR modifies the query_context_modified function to account for this case.

Note that this approach is a short term solution to unblock 4.1. I think two better (but more complext) solutions would be:

1. Make sure the query object from an unmodified chart in a dashboard always matches the stored query object.
2. Even better: dashboards should send the chart ID and any native filters, instead of sending the whole payload for the chart. This would prevent maliciously modifying requests over the wire.

Fixes #30205

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[rusackas]

Yaaaaay! It's almost 4.1 go-time! 🚀