Logs Panel: Add CSV to download options

[gtk-grafana]

What is this feature?
Adding CSV option to logs download.

Why do we need this feature?
We got this feedback quite a bit from the logs table feedback form while it was in public preview.
While there is a CSV download in the query inspector the labels aren't extracted so it's not really helpful, also it's not anywhere near the other download options in the UI so I get that users are confused.

Who is this feature for?
Users of logs in explore.

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.

[ivanahuckova]

This is really good! I think we should process all logRows though, not just the first one - mainly with logs viz. With table only, I wouldn't mind it, as we are selecting there rows, but there we are not showing selected logs, but the first logs.

[ivanahuckova]

@gtk-grafana the tests seems to be failing. Otherwise lgtm.