URL editor improvements

[streamich]

Create a more powerful and user-friendly text input for editing URLs with interpolated Handlebars variables. This should probably be Monaco editor with custom written or modified mode.

Possible features:

• Syntax highlighting for Handlebars variables.
• Syntax highlighting for different URL parts.
• Autocompletion for Handlebars variables.
• Description text for each variable inside autocomplete popup.
• Show autocomplete popup on Handlebars variable mouse click/hover.

👆 Those are just preliminary ideas. Maybe some of the above would be hard to impossible to achieve.

Q: Maybe this should be a EUI component? Similar to markdown editor: https://elastic.github.io/eui/#/editors-syntax/markdown-editor

Q: Maybe this should be related to custom api actions builder? @Dosant: to link an issue

Part of #55324

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[Dosant]

POC with EUICodeEditor for suggestions and some highlighting:
https://codesandbox.io/s/heuristic-elbakyan-v7n1x?file=/index.js

[Dosant]

POC used EUICodeEditor, but we should try newer MonacoEditor.