Add datasource for hosting channel (#7079) (#13686)

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

.changelog/7079.txt

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+```release-note:new-datasource
+`google_firebase_hosting_channel`
+```

google/data_source_google_firebase_hosting_channel.go

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+package google

google/data_source_google_firebase_hosting_channel_test.go

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+package google

website/docs/d/firebase_hosting_channel.html.markdown

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+---
+subcategory: "Firebase"
+page_title: "Google: google_firebase_hosting_channel"
+description: |-
+  A Google Cloud Firebase Hosting Channel instance
+---
+
+# google\_firebase\_hosting\_channel
+
+A Google Cloud Firebase Hosting Channel instance
+
+~> **Warning:** This resource is in beta, and should be used with the terraform-provider-google-beta provider.
+See [Provider Versions](https://terraform.io/docs/providers/google/guides/provider_versions.html) for more details on beta resources.
+
+
+## Argument Reference
+
+The following arguments are supported:
+
+* `site_id` - 
+  (Required)
+  The ID of the site this channel belongs to.
+
+* `channel_id` - 
+  (Required)
+  The ID of the channel. Use `channel_id = "live"` for the default channel of a site.
+
+## Attributes Reference
+
+In addition to the arguments listed above, the following computed attributes are exported:
+
+* `id` - An identifier for the resource with format `sites/{{site_id}}/channels/{{channel_id}}`. Same as `name`
+
+* `name` - The fully-qualified resource name for the channel, in the format: `sites/{{site_id}}/channels/{{channel_id}}`.