Add gzip and security header for documentation and API endpoints

[edlerd]

Required information

• Distribution: snap
• Distribution version: 5.21.0-0cbd19b

Issue description

The documentation served with LXD needs the same security headers as the UI received in #13100

Both documentation and all API endpoints should have support for gzip. Especially the 1.0/metadata/configuration and documentation/objects.inv.txt paths have large response payloads and would benefit from compression.

Steps to reproduce

Documentation

1. Request local path GET /documentation
2. Check security headers are present and gzip compression is enabled in the response

API endpoints

1. Request API endpoint GET 1.0/metadata/configuration
2. Check the response has gzip compression enabled.

[gabrielmougard]

@edlerd thanks

[gabrielmougard]

@edlerd updated