InfluxDB: Interpolate retention policies

[itsmylife]

What is this feature?

In the current state of InfluxDB data source, we are not interpolating the retention policies.
This PR is addressing that.

Why do we need this feature?

To be able to use retention policies as variables.

Who is this feature for?

InfluxDB users

Which issue(s) does this PR fix?:

Fixes #67602

How to test

• Create a dashboard with a variable with the query SHOW RETENTION POLICIES
• In the panel select the variable in the retention policy dropdown
• In the network tab observe the query call. See that the selected retention policy is being used.
• Change the retention policy variable. Observe the new retention policy is being used in the query

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.

[grafanabot]

Hello @itsmylife!
Backport pull requests need to be either:

• Pull requests which address bugs,
• Urgent fixes which need product approval, in order to get merged,
• Docs changes.

Please, if the current pull request addresses a bug fix, label it with the type/bug label.
If it already has the product approval, please add the product-approved label. For docs changes, please add the type/docs label.
If the pull request modifies CI behaviour, please add the type/ci label.
If none of the above applies, please consider removing the backport label and target the next major/minor release.
Thanks!

[grafanabot]

Hello @itsmylife!
Backport pull requests need to be either:

• Pull requests which address bugs,
• Urgent fixes which need product approval, in order to get merged,
• Docs changes.

Please, if the current pull request addresses a bug fix, label it with the type/bug label.
If it already has the product approval, please add the product-approved label. For docs changes, please add the type/docs label.
If the pull request modifies CI behaviour, please add the type/ci label.
If none of the above applies, please consider removing the backport label and target the next major/minor release.
Thanks!

[bohandley]

LGTM!