docs: Create main.tf to support quickstart in Eventarc docset

[camiekim]

Per b/355941670

Create new Terraform quickstart for Eventarc

Description

Fixes #

Note: If you are not associated with Google, open an issue for discussion before submitting a pull request.

Checklist

Readiness

• [ x] Yes, merge this PR after it is approved
• No, don't merge this PR after it is approved

Style

• [ x] My sample follows the rules described for Terraform in the Effective Samples style guide
• [ x] My sample follows the other requirements and best practices in the Contributing
  guide

Testing

• [ x] I have performed tests described in the Contributing guide:

  • [x ] Tests pass: terraform apply
  • [ x] Lint pass: terraform fmt check

Intended location

• [x ] Yes, this sample will be (or already is) included on cloud.google.com
  Location(s): https://cloud.devsite.corp.google.com/eventarc/standard/docs/run/create-trigger-pub-sub-terraform

• No, this sample won't be included on cloud.google.com
  Reason:

API enablement

• If the sample needs an API enabled to pass testing, I have added the service to the Test setup file

Review

• If this sample adds a new directory, I have added codeowners to the CODEOWNERS file