* Make offchain tx pool creation reusable
Introduces an `OffchainTransactionPoolFactory` for creating offchain transactions pools that can be
registered in the runtime externalities context. This factory will be required for a later pr to
make the creation of offchain transaction pools easier.
* Update client/transaction-pool/api/src/lib.rs
Co-authored-by: Anton <anton.kalyaev@gmail.com>
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Co-authored-by: Anton <anton.kalyaev@gmail.com>