Move cloudwatch's copied `instrumentContext` into grafana-plugin-sdk-go

[njvrzm]

What is the chore?: In order to keep log output the same when decoupling cloudwatch from pkg/infra/log I copied the instrumentContext method from that package into cloudwatch.go. This should be in the sdk instead so it can be used by other datasources. It's not entirely clear to me where that leaves the existing instrumentContext and the associated ContextualLoggerMiddleware, so this needs some discussion.

[grafanabot]

@njvrzm please add one or more appropriate labels. Here are some tips:

• if you are making an issue, TODO, or reminder for yourself or your team, please add one label that best describes the product or feature area. Please also add the issue to your project board. :rocket:

• if you are making an issue for any other reason (docs typo, you found a bug, etc), please add at least one label that best describes the product or feature that you are discussing (e.g. area/alerting, datasource/loki, type/docs, type/bug, etc). Our issue triage doc also provides additional guidance on labeling. :rocket:

  Thank you! :heart: