bdk_hwi, set cargo bdk version to 1.0.0-alpha.8

[notmandatory]

Description

Need to set a specific version for bdk dependency in order to be able to publish bdk_hwi to crates.io.

cargo publish -p bdk_hwi
    Updating crates.io index
error: all dependencies must have a version specified when publishing.
dependency `bdk` does not specify a version
Note: The published dependency will use the version from crates.io,
the `path` specification will be removed from the dependency declaration.

Checklists

All Submissions:

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