Add ability to create RPC service chain

We want to update the `fetch` middleware in `eth-json-rpc-middleware`
and the Infura middleware in `eth-json-rpc-infura` to automatically fail
over to alternate RPC endpoints when the desired endpoint is down. To do
this, we need a way to string together a collection of RPC services,
where the first service represents the primary endpoint, the second
service is failover for the first, the third service is a failover for
the second, etc. This composite object should conform to the same
interface as an RPC service so consumers do not have to care whether
they are working with a single endpoint or a chain of them.

Also expose `AbstractRpcService` so that we can make use of it in the
middleware packages.

packages/network-controller/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Add `AbstractRpcService`, the interface for all RPC services ([#5226](https://github.com/MetaMask/core/pull/5226))
+
 ### Changed
 
 - Bump `@metamask/base-controller` from `^7.0.0` to `^7.1.0` ([#5079](https://github.com/MetaMask/core/pull/5079))

packages/network-controller/src/index.ts

@@ -52,3 +52,4 @@ export type {
 } from './types';
 export { NetworkClientType } from './types';
 export type { NetworkClient } from './create-network-client';
+export type { AbstractRpcService } from './rpc-service/abstract-rpc-service';