Add a function to return the version of BDK at runtime

[afilini]

Description

This can be used in a few different ways:

• For dynamically-linked libraries this can be used to determine at runtime if the version of BDK is compatible, i.e. if it's the same an executable was linked against originally
• We can print this in bdk-cli or other tools

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature
• I've updated CHANGELOG.md

[notmandatory]

ACK df32c84. I'll add a bdk-cli command for it. 😃