Cloudwatch: Add back support for old Log Group picker

[sarahzinger]

What is this feature?

When we Added Cross Account Observability to Cloudwatch Logs, we switched to a new log group picker.

This log group picker lets users select Log Groups across multiple accounts if available, and rather than save them by Log Group Name, we saved them by ARN, because Log Group Names do not have to be unique across aws accounts.

After some testing, we decided it seemed to be working well for users using Cross Account Observability and users who were not so we switched all users to the new Log Group Picker.

Unfortunately we learned after doing so that the aws api to query across log groups using log group arns is not supported in Gov Cloud just yet, and may not for sometime. This PR brings back the old Log Group Picker. Most users will continue to see the new Log Group Picker, however if users would like to revert to the old one, they now can by setting a feature flag in their custom.ini (or by making a support request if they are in Grafana Cloud)

[feature_toggles]
cloudWatchCrossAccountQuerying = false

Why do we need this feature?

To let AWS Gov Cloud upgrade to more recent versions of Grafana while still using Cloudwatch Logs.

Who is this feature for?

AWS Gov Cloud users

Which issue(s) does this PR fix?:

Fixes #71982

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.

[kevinwcyu]

is there a reason this is a var on this line instead of it being on line 310?

response, err := logsClient.DescribeLogGroups(input)

[sarahzinger]

no reason I can think of! I just copy and 🍝 from what we used to have but I like the suggestion! Changing now :)

[iwysiu]

Looks good! I assume you tried it with a CloudWatch query that returns data (turns out I don't have one locally)