[cloud provider] Add AWS EC2 instance id semantic convention

[mx-psi]

Changes

Add semantic convention for AWS EC2 instance id. This is similar to #15, and is inspired by #576, to ensure that the EC2 instance id can be correctly identified despite ambiguity in the host.id convention meaning.

Note: if the PR is touching an area that is not listed in the existing areas, or the area does not have sufficient domain experts coverage, the PR might be tagged as experts needed and move slowly until experts are identified.

Merge requirement checklist

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