Expose types from `sc-service` (#5855)

# Description

At moonbeam we have worked on a `lazy-loading` feature which is a client
mode that forks a live parachain and fetches its state on-demand, we
have been able to do this by duplicating some code from
`sc_service::client`. The objective of this PR is to simplify the
implementation by making public some types in polkadot-sdk.

- Modules:
- `sc_service::client` **I do not see a point to only expose this type
when `test-helpers` feature is enabled**

## Integration

Not applicable, the PR just makes some types public.

## Review Notes

The changes included in this PR give more flexibility for client
developers by exposing important types.