Extract PartialComponents into a type alias (#2767)

Pulling PartialComponents into a named `Service` type stops clippy
complaining about type complexity and also makes the type signatures a
little less scary to read.

There's two instances where we can't pull it out into a type because a
nightly feature has not yet been stabilised (E.g. the `imp Fn` isn't
stabilised in a type alias context here:
https://github.com/paritytech/polkadot-sdk/blob/d84e135bbfc366a17bb6b72d0fc9d09ee781ab8d/polkadot/node/service/src/lib.rs#L477
).

---------

Co-authored-by: Bastian Köcher <git@kchr.de>

prdoc/pr_2767.prdoc

@@ -0,0 +1,17 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: Extract PartialComponents into type alias `Service`
+
+doc:
+  - audience: Node Dev
+    description: |
+      Simplifies service definitions by extraction of a complicated type into a type alias. No breaking changes.
+
+crates:
+  - name: "sc-service"
+  - name: "node-template"
+  - name: "minimal-node"
+  - name: "cumulus-test-service"
+  - name: "polkadot-parachain-bin"
+  - name: "parachain-template-node"