* [frame/im-online] remove `external_addresses` from heartbeats

Users should use DHT for discovering new nodes. The reason for adding external addresses was
unstable work of authority discovery (see https://github.com/paritytech/substrate/issues/2719),
which is now stable. Hence we can safely remove `external_addresses`.

Refs https://github.com/paritytech/polkadot/issues/7181

* remove unused import

* run benchmark

* remove external_addresses from offchain NetworkState

* add missing fn to TestNetwork

* Revert "run benchmark"

This reverts commit a282042c2d6bf8bae2c383f6e2699c3fe2970a3d.

* update weights

* address @bkchr comments

* remove duplicate fn

* cleanup benchmarking.rs

* fix executor tests

* remove peer_id from hearbeat as well

https://github.com/paritytech/substrate/pull/14251#discussion_r1210887220

* remove MaxPeerDataEncodingSize

* change storage value type to `()`

https://github.com/paritytech/substrate/pull/14251#discussion_r1214268931

* scaffold storage migration

* no need to check the type actually

* remove unnecessary types from v0 mod

* add a test for migration

* expose Config types

+ pre_upgrade and post_upgrade working fn

* fix test

* replace dummy type with ConstU32

* add some comments to migration test

* fix comment

* respond to @bkchr comments

* use BoundedOpaqueNetworkState::default

intead of using default for each field