feat(gateway): Implement new supergraphSdl() config option for dynamic gateway updates

gateway-js/CHANGELOG.md

@@ -4,7 +4,24 @@
 
 > The changes noted within this `vNEXT` section have not been released yet.  New PRs and commits which introduce changes should include an entry in this `vNEXT` section as part of their development.  When a release is being prepared, a new header will be (manually) created below and the appropriate changes within that release will be moved into the new section.
 
-- _Nothing yet! Stay tuned._
+- __BREAKING__: Improve `supergraphSdl` configuration option, provide a clean and flexible interface for updating gateway schema on load and at runtime. This PR brings a number of updates and deprecations to the gateway. Previous options for loading the gateway's supergraph (`serviceList`, `localServiceList`, `experimental_updateServiceDefinitions`, `experimental_supergraphSdl`) are all deprecated going forward. The migration paths all point to the updated `supergraphSdl` configuration option.
+
+The most notable change here is the introduction of the concept of a `SupergraphManager` (one new possible type of `supergraphSdl`). This interface (when implemented) provides a means for userland code to update the gateway supergraph dynamically, perform subgraph healthchecks, and access subgraph datasources. All of the mentioned deprecated configurations now either use an implementation of a `SupergraphManager` internally or export one to be configured by the user (`IntrospectAndCompose` and `LocalCompose`).
+
+For now: all of the mentioned deprecated configurations will still continue to work as expected. Their usage will come with deprecation warnings advising a switch to `supergraphSdl`.
+* `serviceList` users should switch to the now-exported `IntrospectAndCompose` class.
+* `localServiceList` users should switch to the similar `LocalCompose` class.
+* `experimental_{updateServiceDefinitions|supergraphSdl}` users should migrate their implementation to a custom `SupergraphSdlHook` or `SupergraphManager`.
+
+Since the gateway itself is no longer responsible for composition:
+* `experimental_didUpdateComposition` has been renamed more appropriately to `didUpdateSupergraph` (no signature change)
+* `experimental_compositionDidFail` hook is removed
+
+`experimental_pollInterval` is deprecated and will issue a warning. Its renamed equivalent is `pollIntervalInMs`.
+
+Some defensive code around gateway shutdown has been removed which was only relevant to users who are running the gateway within `ApolloServer` <=v2.18. If you are still running one of these versions, server shutdown may not happen as smoothly.
+
+[#1246](https://github.com/apollographql/federation/pull/1246)
 
 ## v0.45.1