Add block header retrieve branches

[Odraxs]

Description

Changed the name of the BlockHashRequestBody type as it is a payload used in both BlockHash and BlockHeader, for the retrieve_branches functions.

Fixes #225

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented on my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes