feat: support lazy, queued tenant attaches (#6907)

Add off-by-default support for lazy queued tenant activation on attach.
This should be useful on bulk migrations as some tenants will be
activated faster due to operations or endpoint startup. Eventually all
tenants will get activated by reusing the same mechanism we have at
startup (`PageserverConf::concurrent_tenant_warmup`).

The difference to lazy attached tenants to startup ones is that we leave
their initial logical size calculation be triggered by WalReceiver or
consumption metrics.

Fixes: #6315

Co-authored-by: Arpad Müller <arpad-m@users.noreply.github.com>

pageserver/src/http/openapi_spec.yml

@@ -579,6 +579,12 @@ paths:
         required: false
         schema:
           type: integer
+      - name: lazy
+        in: query
+        required: false
+        schema:
+          type: boolean
+        description: Set to true for attaches to queue up until activated by compute. Eager (false) is the default.
     put:
       description: |
         Configures a _tenant location_, that is how a particular pageserver handles