Add user-agent option for install PWA

[xmha97]

Manifest of some PWAs can't be download with desktop user-agent.

firefoxpwa site install https://www.site.com/manifest.json --profile 01JX9W4AC6P2XTQNC82QENHCMG

Caused by:
    expected value at line 1 column 1

Please add user agent option, for example:

firefoxpwa site install https://www.site.com/manifest.json --profile 01JX9W4AC6P2XTQNC82QENHCMG --user-agent="Mozilla/5.0 (Linux; Android 10; K) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/114.0.0.0 Mobile Safari/537.36"

[ipaqmaster]

Seems related to #691 which has been open for a while with no official comment or action.