feat: add capability to reformat Transactions before broadcasting (#1…

…2936)

## 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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### Author Checklist

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- [ ] added `!` to the type prefix if API or client breaking change
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- [ ] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
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- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
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