Closes https://linear.app/sourcegraph/issue/CORE-211

See https://linear.app/sourcegraph/issue/CORE-100 for a higher-level
view - this is the first proof-of-concept for achieving our migration
strategy to extract Enterprise subscription data out of dotcom while
retaining the existing UI until a future project ships a dedicated
Enterprise Portal UI
(https://linear.app/sourcegraph/project/kr-p-enterprise-portal-user-interface-dadd5ff28bd8).

The integration uses generated ConnectRPC client code + `react-query`,
the latter of which has already been used elsewhere for SSC
integrations. This is partly supported by
https://github.com/connectrpc/connect-query-es which offers
mostly-first-class integration with `react-query`, but I had to do some
fenangling to provide the query clients directly as I can't get the
React provider thing to work. The ConnectRPC clients point to the
proxies introduced in
https://github.com/sourcegraph/sourcegraph/pull/63652 which
authenticates the requests for Enterprise Portal, until we ship
https://linear.app/sourcegraph/project/kr-p1-streamlined-role-assignment-via-sams-and-entitle-2f118b3f9d4c/overview

## Test plan

### Local

First, `sg start dotcom`

Choose a subscription you have locally. Use `psql -d sourcegraph` to
connect to local database, then:

```
sourcegraph=# delete from product_licenses where product_subscription_id = '<local subscription ID>';
DELETE 1
sourcegraph=# update product_subscriptions set id = '58b95c21-c2d0-4b4b-8b15-bf1b926d3557' where id = '<local subscription ID>';
UPDATE 1
```

Now annoyingly the UI will break because there is no license, we need:

```gql
query getGraphQLID {
  dotcom {
    productSubscription(uuid:"58b95c21-c2d0-4b4b-8b15-bf1b926d3557") {
      id # graphQL ID
    }
  }
}

mutation createLicense {
  dotcom {
    generateProductLicenseForSubscription(productSubscriptionID:"<graphQLID>", license:{
      tags:["dev"]
      userCount:100
      expiresAt:1814815397
    }) {
      id
    }
  }
}
```

This effectively lets us have a "pretend S2" subscription locally.
Visiting the subscription page now at
https://sourcegraph.test:3443/site-admin/dotcom/product/subscriptions/58b95c21-c2d0-4b4b-8b15-bf1b926d3557


![image](https://github.com/sourcegraph/sourcegraph/assets/23356519/1e77d77d-8032-436b-ab1d-393b34e8e4b5)

The data matches the "real" data currently at
https://sourcegraph.com/site-admin/dotcom/product/subscriptions/58b95c21-c2d0-4b4b-8b15-bf1b926d3557

### Against dotcom

```
sg start web-standalone
```

follow
https://www.loom.com/share/6cb3b3ca475b4b9392aa4b11938e76e6?sid=6cd1a689-d75d-4133-bcff-b0c7d25b23f1
and then check out some product subscriptions