add custom code support to CLI speed benchmark

[evornov]

Description

Adds --code option to the speed benchmark CLI command so that pipelines with custom components can be benchmarked.
I used [doc.from_docs([doc]) for doc in docs] to deep-copy the warmup docs to prevent errors with pipelines that modify sentence boundaries. Happy to change this to something else if there's a more idiomatic way to copy those docs!

Types of change

CLI enhancement + associated documentation update

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[danieldk]

Thanks for the PR, we forgot this option when adding the benchmark speed subcommand.

[danieldk]

Awesome, thanks again for the PR!