support custom method

[daquexian]

Description (e.g. "Related to ...", etc.)

This PR supports registering a custom method. An example of custom method is https://github.com/MaskRay/ccls/wiki/LSP-Extensions.

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly
• CONTRIBUTORS.md was updated, as appropriate
• Changelog has been updated, as needed (see CHANGELOG.md)

[tombh]

Wow, long time since this received any feedback. Apologies.

I'm wondering how this is different from the "Commands" as described in the docs: https://pygls.readthedocs.io/en/latest/pages/advanced_usage.html#commands

[tombh]

Closing due to inactivity. Happy to re-open if this is still needed.