Add broadcast method on Esplora client

[thunderbiscuit]

This PR adds the broadcast() method on the blocking Esplora client with accompanying tests. I think together with the work of the past few weeks this completes a nice bundle ready for a release. Sync, balance, build a tx, sign.

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

[reez]

ACK 15c1f19