Make Sync Server Url configurable in the UI

[bsclifton]

Description

Originally tracked with #12314

Custom sync URLs are supported. You can use the command line flag:
--sync-url=https://sync-v2.brave.com/v2

And replace https://sync-v2.brave.com/v2 with the server of your choosing. Our server implementation is open and can be found here:
https://github.com/brave/go-sync

You can also use group policy to override the URL. For more information on the group policy implementation, please check out https://support.brave.com/hc/en-us/articles/360039248271-Group-Policy

The ask for this issue is to offer a place in the UI to edit the URL.

[bsclifton]

NOTE: a whole lot of great work was done by @jagadeshjai (please see brave/brave-core#25484).

However, there were some concerns captured in #43128 and this work was ultimately reverted with brave/brave-core#27136

If attempted again, we'll need to do a security review. Any Brave employee can help start that process. I apologize for missing it in the original review 😢

There's also an issue reported by @MadhaviSeelam here:
#43022

Where there can be a freeze if an invalid URL is provided. I closed the issue for now (as feature has been reverted), but we'll want to consider that case if another attempt comes in.

[bsclifton]

Maybe @fmarier or @diracdeltas can comment here

But I think when attempted again, we could remove all the UI code and instead go with a flag. Something like brave://flags/#brave-custom-sync-url (this is just an example).

This should eliminate the complexity with the UI and be a fairly straight-forward change we can use on Desktop and Android.

[bsclifton]

NOTE: an Android specific PR is open with brave/brave-core#26869 which has some great UI. But given the security review feedback on prior approach, we'd be better to offer a flag for this because it's not common and can be confusing for regular users