Add system.memory.limit

[mx-psi]

Updates #127

Changes

Adds system.memory.limit which reports the total available memory in the system.

A reference implementation is available at open-telemetry/opentelemetry-collector-contrib/pull/27096

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• CHANGELOG.md updated for non-trivial changes.
• schema-next.yaml updated with changes to existing conventions.

[mx-psi]

After #89 the total value is listed as a valid attribute for system.memory.state, so system.memory.limit would just be system.memory.usage{system.memory.state=total} and I can close this and just implement this on the Collector.