database: Merge Cosmos DB containers

[mbarnes]

What this PR does

This pull request contains all the breaking changes related to ARO-14170 - Merge Cosmos DB containers.

🚨 Give the ARO-HCP team a heads up before merging this! (auto-merge is disabled) 🚨

In addition to merging the "resources", "operations", and "subscriptions" Cosmos DB containers, this pull request also changes the structure of all Cosmos DB documents in this merged container.

A new typedDocument struct standardizes the structure of documents as follows:

  {
    "id":           <item-id>
    "partitionKey": <partition-key>
    "resourceType": <azure-resource-type>
    "properties": {
      ... content depends on "resourceType" ...
    }
    "_rid":  <system-generated>
    "_self": <system-generated>
    "_etag": <system-generated>
    "_ts":   <system-generated>
  }

A key insight is none of the system-generated fields are used outside of the database package. Therefore the data that the frontend/backend components care about can be encapsulated in a "properties" value, and the other document types no longer need to embed baseDocument.

DocumentProperties is an interface for types that can be plugged into the "properties" field, like ResourceDocument. The interface requires a GetValidTypes() method that returns a set of valid values for the "resourceType" field, used for validation when [un]marshalling.

One case worth noting is SubscriptionDocument was mostly already following this format, with its top-level "subscription" field defined as a arm.Subscription struct:

type SubscriptionDocument struct {                                                              
       baseDocument                                                                             
       Subscription *arm.Subscription `json:"subscription,omitempty"`                           
}                                

With this PR, arm.Subscription now implements the DocumentProperties interface and SubscriptionDocument is gone.

Also worth mentioning is DBClientIteratorItem has been redefined as a iter.Seq2 instead of a iter.Seq. Effectively this means the DBClientIterator.Items method now returns the Cosmos DB item ID separately from the item (document) itself.

To illustrate:

old style:  for doc := range iterator.Items(ctx)
new style:  for id, doc := range iterator.Items(ctx)

Jira: ARO-14170 - Merge Cosmos DB containers
Link to demo recording:

Special notes for your reviewer

[github-actions]

Please rebase pull request.

[github-actions]

Please rebase pull request.

[SudoBrendan]

OK - this review is partial. Major finding is panic risky code - let's fixup the interfaces we actually need for this instead of using T (you arrived there too - just didn't make it into prod code - YAY TESTS!).

Great work as always Matt - I'll find time to review the rest later.

For my future self: My review ended on the first commit - stopped at typeddocument_test.go

Requested changes were addressed.

[mbarnes]

Prepended a new commit to adapt to #1272.

Just when I claim the Cosmos DB system-generated fields aren't needed outside the database package, a case pops up.

[github-actions]

Please rebase pull request.

[mbarnes]

I tacked on yet another commit to undo #1189 - database: Add PartitionKeys container because Microsoft has released partial cross-partition query support in azcosmos v1.3.0. I tested and verified that it works well enough for our purpose. Adding here since it fits with the theme of dropping Cosmos containers.