Add showTestNetworks user pref in Settings

[Douglashdaniel]

Description

We need a showTestNetworks: boolean user pref in brave://settings/wallet to allow users to decide if they want to see Test Networks as an option in their wallet. Could be just a boolean value and the Front-end can filter out those networks or vice versa if we want to handle on the backend.

[srirambv]

Does Android inherit this changes or should that be done separately?

cc: @SergeyZhukovsky @wchen342 @Pavneet-Sing

[SergeyZhukovsky]

@srirambv it's a front-end work, need to be done separately on mobile

[kjozwiak]

The above requires 1.39.110 or higher for 1.39.x verification 👍

[srirambv]

Brave	1.39.111 Chromium: 102.0.5005.61 (Official Build) (64-bit)
Revision	0e59bcc00cc4985ce39ad31c150065f159d95ad3-refs/branch-heads/5005@{#819}
OS	☑️ Linux	☑️ Windows 11 Version 22H2 (Build 22621.1)	☑️ macOS Version 12.0.1 (Build 21C52)
Verified steps from brave/brave-core#13098 Verified new setting called Show test networks is available under brave://settings/wallet/networks Verified setting is disabled by default hence no test network is shown by default Verified enabling the settings shows all test networks for default networks(Solana/Ethereum) Encountered #22953
Show test networks Off
Show test networks On
Test networks shown network selector
Test networks not visible on network selector