feat(example): add CPFP example using TxBuilder

[noahjoeris]

Description

Adds an example demonstrating CPFP using TxBuilder.

Changelog notice

• Add: Example CPFP using TxBuilder (examples/cpfp.rs).

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.30%. Comparing base (4b612f5) to head (0c6bcfc).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #483   +/-   ##
=======================================
  Coverage   80.30%   80.30%           
=======================================
  Files          24       24           
  Lines        5417     5417           
  Branches      245      245           
=======================================
  Hits         4350     4350           
  Misses        989      989           
  Partials       78       78           

Flag	Coverage Δ
rust	80.30% <ø> (ø)

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