Add shell completions command

[va-an]

Describe the enhancement
Add shell completions support via a new completions subcommand, similar to how rustup completions works.
The command would generate shell completion scripts for bash, zsh, fish, powershell, and elvish using clap_complete.

Example usage:

bdk-cli completions bash > ~/.local/share/bash-completion/completions/bdk-cli
bdk-cli completions zsh > ~/.zfunc/_bdk-cli

Use case
bdk-cli has many subcommands and options. Typing out full command names gets tedious, so tab completion would make the CLI much nicer to use.

Additional context
I've already started looking into the implementation and can open a PR if this sounds good.

[AmosOO7]

This is a good one. it will really love to see what you have done once you open a PR