Splunk observability scaler

[sschimper-splunk]

Provide a description of what has been changed

Checklist

• [x] When introducing a new scaler, I agree with the scaling governance policy
• [x] I have verified that my change is according to the deprecations & breaking changes policy
• [x] Tests have been added
• [x] Changelog has been updated and is aligned with our changelog requirements
• [] A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• [x] A PR is opened to update the documentation on (repo) (if applicable)
• [x] Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #

Relates to #