Split the single 'open *all* links in PM' pref into 2 settings: one for 3rd party, one for shortcuts

[klint]

Why/User Benefit/User Problem

Note: this was originally expressed as a post-closure comment in #5341, which was not the right place. It may also be related to #5742 and #514

First, here are 2 assumptions that make sense to me (at least ;) ):

1. Shortcuts to websites on phone's homescreen are a very powerful feature to avoid cluttering the phone with applications, either for PWA-enabled sites or for non PWA-enabled sites.
   This request deals with non PWA-enabled sites only: in this case, and in order to emulate "PWA"-like behaviour, sites are often in a persistent context, login/session/cookie-wise. Therefore they cannot be opened in Private mode, where all initial context would be lost.
   (Yet I understand that some users may want to have their shortcuts always open in Private mode, although that is not my way).

2. On the contrary, there is a need to be able to open 3rd party links in Private mode, as they usually don't need any login/session/cookie context and do not need to be recorded.
   My understanding is that also includes custom tabs, does'nt it?

The problem is that there is only one setting that defines the mode 3rd party links and shortcurt are opened in: "open all links in private mode".

What/Requirements

The requirement is to split the single 'open all links in PM' preference into 2 different settings:

Setting A: 'open all 3rd party links in PM' (which would open all links from other applications, but not Fenix shortcuts)
Setting B: 'open all Fenix shortcuts in PM' that would apply to Fenix page shortcuts on the phone's homescreen only, obviously.

Acceptance Criteria (how do I know when I’m done?)

When setting A is ON and setting B is OFF, a page/custom tab opened by a 3rd party will open in private mode, whereas a page shortcut will open in non-private mode, in full logged in context.
Etc
Etc

┆Issue is synchronized with this Jira Task

[cpeterso]

Moved to bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1807435

Change performed by the Move to Bugzilla add-on.