[resource/host] Define host.mac semantic convention

[mx-psi]

Updates #131

Changes

Add host.mac semantic convention to list MAC addresses of a host.

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]

@open-telemetry/semconv-system-approvers PTAL!

[mx-psi]

@open-telemetry/specs-semconv-maintainers This is ready for a review from maintainers.

The only remaining point of contention is the name, since host.mac could be argued to be singular but its type is an array, similar to the discussion at #203. We don't have a strong opinion on the system semconv WG over whether the plural rule should be followed as opposed to preserving backwards compatibility with ECS, so we believe it is up to maintainers now to decide on this :)

[joaopgrassi]

This is essentially the same discussion as with the host.ip and ECS. I'm inclined to accept it under host.mac for the same reasons the other was accepted.