feat/overseer: introduce closure init (#3775)

* feat/overseer: introduce closure init

Enables removal of the connected/disconnected overseer state.

* feat/overseer: allow replacement logic to access the original

Allows to re-use init-once types, which would otherwise error.

* feat/overseer: introduce external connector

Preparation for removal of `AllSubsystems`
which is another prerequisite for removing
the connect/disconnect state.

* fix/test: replace needs closure

* fixup

* simplify

* mea culpa

* all-subsystems-gen test

polkadot/node/overseer/all-subsystems-gen/tests/ui/err-01-generic-used-twice.rs

@@ -12,5 +12,5 @@ fn main() {
 		a: 0_u16,
 		b: 1_u16,
 	};
-	let _all = all.replace_a(77u8);
+	let _all = all.replace_a(|_| 77u8);
 }

polkadot/node/overseer/all-subsystems-gen/tests/ui/err-01-generic-used-twice.stderr

@@ -10,5 +10,5 @@ error[E0599]: no method named `replace_a` found for struct `AllSubsystems<u16>`
 5  | struct AllSubsystems<X> {
    | ----------------------- method `replace_a` not found for this
 ...
-15 |     let _all = all.replace_a(77u8);
+15 |     let _all = all.replace_a(|_| 77u8);
    |                    ^^^^^^^^^ method not found in `AllSubsystems<u16>`

polkadot/node/overseer/all-subsystems-gen/tests/ui/err-01-no-generic.rs

@@ -13,5 +13,5 @@ fn main() {
 		a: 0_f32,
 		b: 1_u16,
 	};
-	let _all = all.replace_a(77u8);
+	let _all = all.replace_a(|_| 77u8);
 }

polkadot/node/overseer/all-subsystems-gen/tests/ui/err-01-no-generic.stderr

@@ -10,5 +10,5 @@ error[E0599]: no method named `replace_a` found for struct `AllSubsystems` in th
 6  | struct AllSubsystems {
    | -------------------- method `replace_a` not found for this
 ...
-16 |     let _all = all.replace_a(77u8);
+16 |     let _all = all.replace_a(|_| 77u8);
    |                    ^^^^^^^^^ method not found in `AllSubsystems`

polkadot/node/overseer/all-subsystems-gen/tests/ui/err-01-no-generics.stderr

@@ -10,5 +10,5 @@ error[E0599]: no method named `replace_a` found for struct `AllSubsystems<u16>`
 6  | struct AllSubsystems<X> {
    | ----------------------- method `replace_a` not found for this
 ...
-16 |     let _all = all.replace_a(77u8);
+16 |     let _all = all.replace_a(|_| 77u8);
    |                    ^^^^^^^^^ method not found in `AllSubsystems<u16>`

polkadot/node/overseer/all-subsystems-gen/tests/ui/ok-01-w-generics.rs

@@ -13,5 +13,5 @@ fn main() {
 		a: 0u8,
 		b: 1u16,
 	};
-	let _all: AllSubsystems<_,_> = all.replace_a::<u32>(777_777u32);
+	let _all: AllSubsystems<_,_> = all.replace_a::<u32,_>(|_| 777_777u32);
 }