feat: add completions subcommand

[va-an]

Description

Resolves #234.

Added completions subcommand to generate shell completions.

How to test

Install bdk-cli:

cargo install --path . --all-features

See how to install completions for your shell:

bdk-cli completions --help

For example, to install completions for zsh:

bdk-cli completions zsh > ~/.zfunc/_bdk-cli

After restarting the shell, completions should work with Tab.

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

[coveralls]

Pull Request Test Coverage Report for Build 21804713374

Details

• 0 of 16 (0.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 10.686%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/utils.rs	0	1	0.0%
src/error.rs	0	3	0.0%
src/handlers.rs	0	6	0.0%
src/payjoin/mod.rs	0	6	0.0%

Totals
Change from base Build 21153868360:	-0.03%
Covered Lines:	268
Relevant Lines:	2508

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💛 - Coveralls