Add event_filters to event triggers in cloudfunctions2 resource

[modular-magician]

Description

Closes hashicorp/terraform-provider-google#12021

This PR adds the ability to set event filters within event triggers defined when creating a 2nd generation cloud function. Previously users could make 2nd gen functions be triggered by events sent from an 'event provider' (i.e. a Google API) within Eventarc, but now they can filter those events based on the event's attributes.

Valid attributes that can be used in filters differ by the event provider - here's the relevant part of the docs that describes a gcloud command to learn more about the events and attributes for a given event provider.

E.g. All events related to the storage.googleapis.com event provider have attributes type and bucket - and bucket wouldn't be an attribute on non-GCS events

$ gcloud eventarc providers describe storage.googleapis.com --location=us-central1

displayName: Cloud Storage
eventTypes:
- description: The live version of an object has become a noncurrent version.
  filteringAttributes:
  - attribute: bucket
    description: The bucket name being watched.
    required: true
  - attribute: type
    required: true
  type: google.cloud.storage.object.v1.archived

#...omitted other events in list...

name: projects/[PROJECT_ID]/locations/us-central1/providers/storage.googleapis.com

Things to note

1. How should operator be used? I couldn't work it out for an acceptance test - addressed in fb04576 - docs are here and path patterns aren't appropriate for all attributes

2. The type attribute cannot be used in the new event_filters block because it is already set by the event_type field inside the event_trigger block.

A configuration similar to the one below causes this error

The request was invalid: duplicated declaration of attribute \"type\" in trigger.event_filters

resource "google_cloudfunctions2_function" "terraform-test" {
 
  ...

  event_trigger {
    trigger_region                = "us-central1"
    event_type                    = "google.cloud.storage.object.v1.finalized"
    retry_policy                  = "RETRY_POLICY_RETRY"
    service_account_email = google_service_account.account.email
    event_filters {
      attribute                        = "bucket"
      value                            = google_storage_bucket.trigger-bucket.name
    }
    event_filters {
      attribute                        = "type" # This clashes with event_type and triggers an API error
      value                            = "google.cloud.storage.object.v1.finalized"
    }
  }
}

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

cloudfunctions2: added field `event_filters` to resource `google_cloudfunctions2_function` (beta)

Derived from GoogleCloudPlatform/magic-modules#6278