Store the impersonator id on access tokens created via ZED_IMPERSONATE

* Use the impersonator id to prevent these tokens from counting
  against the impersonated user when limiting the users' total
  of access tokens.
* When connecting using an access token with an impersonator
  add the impersonator as a field to the tracing span that wraps
  the task for that connection.
* Disallow impersonating users via the admin API token in production,
  because when using the admin API token, we aren't able to identify
  the impersonator.

Co-authored-by: Marshall <marshall@zed.dev>

crates/collab/migrations/20240117150300_add_impersonator_to_access_tokens.sql

@@ -0,0 +1,3 @@
+ALTER TABLE access_tokens ADD COLUMN impersonator_id integer;
+
+CREATE INDEX "index_access_tokens_impersonator_id" ON "access_tokens" ("impersonator_id");