[8.x](backport #5535) [Helm] Implement values.schema.json

[mergify]

What does this PR do?

This PR does the following:

• introduces a value.schema.json inside the chart which is used by Helm to validate the input values.yaml
• splits ESPlainAuth output type into ESPlainAuthBasic and ESPlainAuthAPI types, favouring explicitness, for better validation and
• introduces hostNetwork as an option in an Agent preset

Why is it important?

By having a value.schema.json we can reduce validation checks inside the templates of the Helm chart (less ifs inside templates). Furthermore, many text editors/IDEs offer the capability to import the schema and hence get hints and proposal write writing a values.yaml for the Helm chart

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

N/A

How to test this PR locally

Checking validation

run the following which is actually missing required values

helm template test ./deploy/helm/elastic-agent --set kubernetes.enabled=true  --set outputs.default.type=ESSecretAuthBasic  --set agent.fleet.enabled=true --set agent.fleet.token=""

you should see the following errors

Error: values don't meet the specifications of the schema(s) in the following chart(s):
elastic-agent:
- agent.fleet: Must validate "then" as "if" was valid
- agent.fleet: Must validate at least one schema (anyOf)
- agent.fleet.url: Does not match format 'uri'
- agent.fleet.kibanaHost: Does not match format 'uri'
- agent.fleet.kibanaUser: String length must be greater than or equal to 1
- agent.fleet.kibanaPassword: String length must be greater than or equal to 1
- outputs.default: Must validate "then" as "if" was valid
- outputs.default.secretName: String length must be greater than or equal to 1
- outputs.default: Must validate all the schemas (allOf)

Checking hints

1. Create a new values.yaml file with your text editor/IDE (tested with vscode)
2. Add on top this comment

   # yaml-language-server: $schema=elastic-agent/deploy/helm/elastic-agent/values.schema.json

3. For any fields you add below the above line you should get hints

Related issues

N/A

---

This is an automatic backport of pull request #5535 done by [Mergify](https://mergify.com).

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[mergify]

This pull request has not been merged yet. Could you please review and merge it @pkoutsovasilis? 🙏

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarQube