Add support for external `clap` subcommands in hugr-cli

[ss2165]

The hugr-cli crate is a installable binary for manipulating and verifying HUGR programs from the command line.
See package documentation here: https://docs.rs/hugr-cli/latest/hugr_cli/
The source code can be found under the /hugr-cli/ subdirectory.

The binary currently provides two commands,

• hugr validate will validate a .hugr file and report any errors.
• hugr mermaid will render the HUGR as a mermaid graph.

We want to extend the functionality via external tool in a similar manner to how cargo does it,
If a custom tool foo is available in the user's PATH as hugr-foo, calling hugr foo will call it directly and forwards arguments to it.

The crate uses clap for handling arguments. Some hooks for external commands have been defined already. See:

• Cli arguments definition, including an external_subcommand variant: https://github.com/CQCL/hugr/blob/b162a4fe4135e25055d9393da5497c74a997d21d/hugr-cli/src/lib.rs#L85-L86

• Cli argument handling, currently panics: https://github.com/CQCL/hugr/blob/b162a4fe4135e25055d9393da5497c74a997d21d/hugr-cli/src/main.rs#L14-L19

• Clap's documentation about external subcommands: https://docs.rs/clap/latest/clap/struct.Command.html#method.allow_external_subcommands
  Note the remark about needing to check for subcommand existence and giving an appropriate error if necessary.

• Our development guide for setting up your environment: DEVELOPMENT.md

What we expect from a PR:

• Support for calling external tools from a user's PATH.
• Some simple tests for error handling (see /hugr-cli/tests/)
• Green CI checks for formatting, clippy lints, and tests.

Bonus points:

• Add an integration test with a dummy tool.

[aborgna-q]

@OsamaMIT Can you add a comment on this Issue, so I can assign it to you?