Add support for definitions related to deployment modes

[jsoriano]

What does this PR do?

Add support for definitions related to deployment modes. It adds two main features:

• Policy templates can indicate on what deployment modes they can be used.
• Variables can be hidden per deployment mode.

It will be released in two versions:

• 3.1.4 will add support for agentless mode.
• 3.2.0 will add settings for default mode.

default mode is introduced in the next minor because it will need changes in the stack to be properly supported, while the changes for agentless will merely be ignored in stacks that don't support it.

Why is it important?

With the introduction of new deployment modes we need ways to define policy templates or use variables only on certain deployment modes.

Checklist

• [x] 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

• Part of https://github.com/elastic/package-spec/issues/684

[elasticmachine]

:green_heart: Build Succeeded

• Buildkite Build
• Commit: d239d1d85da5e788e06ea1a7a07a111e4d368781

cc @jsoriano