Adds top level `volumes` property and `volume` definition

[Tacklebox]

What does this PR do?

This adds the properties and definitions required to support validating docker compose 2.3 volumes in the custom-agent.yml of an elastic integration.

Why is it important?

The cloud_defend integration requires mounting a volume for its system test. The current schema validation fails due to not having definitions for volumes in the custom-agent.yaml file.

Checklist

• [ ] I have added test packages to test/packages that prove my change is effective.
• [x] I have added an entry in spec/changelog.yml.

Related issues

• Relates https://github.com/elastic/integrations/pull/8567

[elasticmachine]

:green_heart: Build Succeeded

• Buildkite Build
• Commit: 64a457211e3369337ce455921cd6832e137eaa95

History

• :green_heart: Build #471 succeeded 4d32890402959186bcddb6dab8ae78dfb0a45d97
• :green_heart: Build #469 succeeded 00802dda69cc34cfe5b78878dab58b062584490b
• :broken_heart: Build #468 failed c9245bf7284a1dea340994e20a83bba63b0422ae