feat: configure project-id and project-number

[v1v]

Notifies elastic/elastic-agent#7232

What

Customise the project-id and project-number so we can support different GCPs.

Test

I'm testing this out in Buldkite, see build [it's not accessible for everyone but only elastic employees]

PR Overview

This PR adds two new configuration properties for GCP integration: project-id and project-number.

• Extends the plugin configuration file to include project-id (string) and project-number (number).
• Updates the README documentation to reflect these new properties in the properties table.

Reviewed Changes

File	Description
plugin.yml	Adds new GCP properties with specified types.
README.md	Updates property table to document the new props.

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

README.md:10

• [nitpick] The ordering of properties in the README table differs from the plugin configuration (project-number is listed before project-id, while plugin.yml defines project-id first). Consider reordering the README rows for consistency.

| `project-number` | The GCP project number.                                                                               | `false`  | `8560181848`            |