[plugin] Public assets no longer exposed

[fastlorenzo]

Kibana version: 7.10.0

Elasticsearch version: 7.10.0

Server OS version: Ubuntu 20.04

Browser version: Any

Browser OS version: Windows 10, Ubuntu 20.04

Original install method (e.g. download page, yum, from source, etc.): apt and from sources (tried both)

Describe the bug: In previous Kibana versions (7.9.2, 7.9.3), I was serving assets (images, html files, etc) by putting them in the public/assets folder of the Kibana plugin I was building. The assets where then available at /plugins/{PLUGIN_ID}/assets/{PATH}.
Since 7.10.0, trying to access the files returns a 404.

Steps to reproduce:

1. Create a plugin
2. Add assets in public/assets
3. Build the plugin and install the zip in a clean Kibana install

Expected behavior: The browser should return the asset

Screenshots (if relevant): N/A

Errors in browser console (if relevant): N/A

Provide logs and/or server output (if relevant):

Example error log when accessing the asset:

{"type":"response","@timestamp":"2020-11-15T00:33:16Z","tags":[],"pid":6,"method":"get","statusCode":404,"req":{"url":"/plugins/redelk/assets/redelklogo-light.svg","method":"get","headers":{"x-real-ip":"172.18.0.1","x-forwarded-for":"172.18.0.1","x-forwarded-proto":"https","x-forwarded-host":"localhost","x-forwarded-port":"443","connection":"upgrade","host":"localhost","upgrade-insecure-requests":"1","user-agent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.198 Safari/537.36","accept":"text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9","sec-fetch-site":"none","sec-fetch-mode":"navigate","sec-fetch-dest":"document","accept-encoding":"gzip, deflate, br","accept-language":"en-US,en;q=0.9"},"remoteAddress":"172.18.0.6","userAgent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.198 Safari/537.36"},"res":{"statusCode":404,"responseTime":26,"contentLength":9},"message":"GET /plugins/redelk/assets/redelklogo-light.svg 404 26ms - 9.0B"}

Any additional context: Plugin source code available at https://github.com/fastlorenzo/redelk-kibana-app

[fastlorenzo]

Might be related to this line:

kibana/packages/kbn-plugin-helpers/src/tasks/write_server_files.ts

Line 61 in bd08439

'**/public/**',

[fastlorenzo]

Temporary workaround here: #75019 (comment)

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)