feat: allow setting of global fallback user agent

[MarshallOfSound]

Currently we support letting apps set user agents on webContents and session but not "for everything". Well this PR allows you to do just that. In particular it allows apps to do a few things

• Set the user agent in one place for everything regardless of sessions / webContents used
• Set the user agent for parts of Chromium that we previously couldn't override due to incomplete wiring inside Chromium. E.g. Tracing controllers, accessibility, service workers, etc.

Notes: Added new app.userAgentFallback property to allow setting the apps useragent globally

[MarshallOfSound]

Put this on the @electron/wg-releases agenda to discuss target/6-0-x

[codebytere]

It'd be awesome if this was set as a property on the prototype instead :)

[codebytere]

--- a/atom/browser/api/atom_api_app.cc
+++ b/atom/browser/api/atom_api_app.cc
@@ -1456,7 +1456,8 @@
 #if defined(OS_MACOSX)
       .SetProperty("dock", &App::GetDockAPI)
 #endif
-      .SetProperty("userAgentFallback", &App::GetUserAgentFallback, &App::SetUserAgentFallback)
+      .SetProperty("userAgentFallback", &App::GetUserAgentFallback,
+                   &App::SetUserAgentFallback)
       .SetMethod("enableSandbox", &App::EnableSandbox);
 }

lint death ☝️

[codebytere]

lgtm but i'd love a second review before we merge this.

[release-clerk]

Release Notes Persisted

Added new app.userAgentFallback property to allow setting the apps useragent globally

[trop]

I was unable to backport this PR to "6-0-x" cleanly;
you will need to perform this backport manually.

[trop]

A maintainer has manually backported this PR to "4-2-x", please check out #18110

[trop]

A maintainer has manually backported this PR to "4-2-x", please check out #18110

[trop]

A maintainer has manually backported this PR to "6-0-x", please check out #18473