[Bridges] Move chain definitions to separate folder (#3822)

Related to
https://github.com/paritytech/parity-bridges-common/issues/2538

This PR doesn't contain any functional changes. 

The PR moves specific bridged chain definitions from
`bridges/primitives` to `bridges/chains` folder in order to facilitate
the migration of the `parity-bridges-repo` into `polkadot-sdk` as
discussed in https://hackmd.io/LprWjZ0bQXKpFeveYHIRXw?view

Apart from this it also includes some cosmetic changes to some
`Cargo.toml` files as a result of running `diener workspacify`.

bridges/primitives/chain-polkadot/Cargo.toml

@@ -1,35 +0,0 @@
-[package]
-name = "bp-polkadot"
-description = "Primitives of Polkadot runtime."
-version = "0.5.0"
-authors.workspace = true
-edition.workspace = true
-license = "GPL-3.0-or-later WITH Classpath-exception-2.0"
-
-[lints]
-workspace = true
-
-[dependencies]
-
-# Bridge Dependencies
-
-bp-header-chain = { path = "../header-chain", default-features = false }
-bp-polkadot-core = { path = "../polkadot-core", default-features = false }
-bp-runtime = { path = "../runtime", default-features = false }
-
-# Substrate Based Dependencies
-
-frame-support = { path = "../../../substrate/frame/support", default-features = false }
-sp-api = { path = "../../../substrate/primitives/api", default-features = false }
-sp-std = { path = "../../../substrate/primitives/std", default-features = false }
-
-[features]
-default = ["std"]
-std = [
-	"bp-header-chain/std",
-	"bp-polkadot-core/std",
-	"bp-runtime/std",
-	"frame-support/std",
-	"sp-api/std",
-	"sp-std/std",
-]