InfluxDB: Introduce custom variable support

[itsmylife]

What is this feature?

In general variable queries are just strings. But we need more flexibility to support various scenarios. To achieve that we need custom variable support. In this PR I am just implementing the custom variable support and not changing any logic.
This will help us to handle/deliver this #84120

Why do we need this feature?

More flexibility in the variable editor

Who is this feature for?

InfluxDB users

How to test

• Have Influxql, Flux and SQL variables in legacy support.
• Switch to this branch and test them again, try to run, refresh, update.
• Introduce a new set of Influxql, Flux and SQL variables and save the dashboard.
• Refresh, view and edit.
• All actions must work.

[NWRichmond]

I followed the testing steps and got the expected result:

demo.influxdb.variables.influxql.sql.flux.mov

DISCLAIMER: With Flux, I couldn't figure out how to get a Query-type variable to work, seemingly because Flux expects time bounds (|> range(start: v.timeRangeStart, stop: v.timeRangeStop)), and dashboard query variables don't appear to support that. So the Flux-derived panels instead use Custom-type variables.

In the process of reviewing this PR, I ran into a bug and created a new issue: #88008.

Nicely done!