[PLUGINS] Add metadata schema examples for common input field types

[utksh1]

Summary

Contributors adding plugins need examples for common field types in metadata.json. Add documented snippets for text, URL, select, boolean, number, and multiselect fields.

Scope

• Update PLUGINS.md.
• Use realistic but generic examples.
• Mention validation-related fields if supported by the current metadata format.

Acceptance Criteria

• Examples are copy-paste friendly but clearly illustrative.
• Existing plugin schema terminology is used consistently.
• No plugin behavior changes.

[SHRESTHA-012]

Hi @utksh1 , I'd like to work on this issue under GSSoC 2026 . Please assign it to me.
I have read the scope properly and would like to work on this and improve my assessing skills.
I would contribute to update the PLUGINS.md file keeping all the necessary instructions in mind.

[SHRESTHA-012]

@utksh1 Thanks for assigning this to me. I’ll start working on it and submit a PR soon.

[utksh1]

Closing as completed by the merged plugin metadata schema documentation updates in #176 and #187.