Explore: Init with mixed DS if there's no root DS in the URL and queries have multiple datasources

[Elfo404]

What is this feature?

Select mixed datasource when accessing Explore with a URL that has not root level datasource defined but multiple queries with different datasources.
Although is not technically possible to generate such URLs from explore itself, this should be the correct behaviour if URLs are tampered with/generated externally.

Previously, if opening Explore with such a URL, the root level DS was set to the first query's DS and all other queries discarded, with this change the "mixed" ds is correctly inferred and no query is is discarded (as long as they have a valid datasource)

Why do we need this feature?

More robust URL parsing and state initialization in Explore.l

Who is this feature for?

Explore users

Which issue(s) does this PR fix?:

Fixes #69689

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

How to test this

Given how unique ids are generated for datasources i can't provide premade links, however this should be quite easy to test:

• navigate to explore, select mixed datasource and set queries with 2 (or more) different datasources.
• copy the URL and remove the root level datasource property from it
• Navigate to that URL. before this change you'd see only queries having the datasource equal to the first query's datasource, and the pane datasource set to said DS. with this change you should see mixed.

[ifrost]