feat(main): Support Go AutoMemLimit

[aryanxgupta]

What this PR does:
Hi,
This PR implements the automemlimit feature requested in issue #7064.

I've followed the Prometheus implementation as a reference, adding the simple SetGoMemLimitWithOpts call at the start of cmd/cortex/main.go and using the project's logger. I did not add the complex flag/config logic which was mentioned in the reference PR.

Note on Tests:
I ran make test locally. The build succeeded and the tests for cmd/cortex passed. However, the test run failed in pkg/alertmanager and pkg/compactor. These appear to be unrelated, so I'm relying on the CI to run the full suite.

Which issue(s) this PR fixes:
Fixes #7064

Checklist

• Tests updated (No new tests were needed for this change)
• Documentation added (No documentation changes were needed)
• CHANGELOG.md updated - I'm not sure if a changelog entry is needed for this. Please let me know if I should add one

[yeya24]

Thanks @aryanxgupta. I think this approach works but this behavior should be behind a feature flag, similar to what Prometheus does. We need a flag to enable auto memlimit and another flag to set auto memlimit ratio.