link_utils: exposing whitelisted protocols to user settings

[shubham-padia]

Fixes #1284.
Adding config option to set protocols in the config that are whitelisted to be opened directly.
The behaviour is documented in docs\howto\customize-link-protocols.md.

Supersedes #1401. Keeps the original author same.

Fixes:

Screenshots and screen captures:

Platforms this PR was tested on:

• Windows
• macOS
• Linux (specify distro)

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[alya]

We should be using standard Zulip capitalization throughout (sentence case).

[alya]

Suggested change

	Note: Each protocol should include the trailing colon (:), e.g., "mailto:" instead of "mailto".
	Note: Each protocol should include the trailing `:`, e.g., "mailto:" instead of "mailto".

[alya]

@andersk Tagging for your review.