## Description
Increase customized signing capabilities by:
- Adding a hook to preprocess transactions before broadcasting
This PR introduces the ability to preprocess transactions before broadcasting, a feature that provides the ability to add extensions, or otherwise customize the payload to satisfy a chain's requirements. For adaptability, the parameters include the keytype to allow differentiation between formatting transactions for Ledger vs. software keys.
## PR
This PR introduces a hook to preprocess transactions, chiefly to add extensions, from a higher-level chain. Key changes and relevant files:
- Add `PreprocessTxFn` type and fields to both `Context` and `Factory`
- Instantiate the `Factory` with the `PreprocessTxFn` provided by the `Context`
- Call `PreprocessTx` with the appropriate parameters after signing
Closes: #XXXX
---
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