Add optional icon field to plugin manifest

[aaronpowell]

Summary

Adds an optional icon field to the plugin manifest so hosts can display a
visual identifier for a plugin in UIs.

Closes #18

Behavior

• icon is optional; if present it MUST be either a plugin-relative path
  (§4.1, begins with ./, resolves within the plugin root) or an absolute
  https: URL with no user information or fragment.
• The image SHOULD be square and at least 512x512 pixels.
• Retrieving/rendering the icon is host-defined; hosts MAY ignore it and MUST
  NOT retrieve a remote icon while loading a plugin (§5.2).
• icon is advisory: an invalid value MUST be ignored and MUST NOT affect
  component discovery or execution.

Changes

• Add icon to the JSON schema (schemas/1.0.0/plugin.schema.json).
• Document icon in the metadata field table (§5.4), the full example manifest,
  and a dedicated §5.7 subsection.

Example

{
  "$schema": "https://open-plugins.com/schemas/1.0.0/plugin.schema.json",
  "name": "plugin-name",
  "icon": "./icon.png",
  "version": "1.2.0"
}