[Desktop] Move wallets business logic in shared lib and split it per providers

[NejcZdovc]

Right now we have in prefs external_wallets where we currently store uphold wallet. Uphold wallet is currently dictionary. All parsing is done in rewards_service_impl. We need to move this into shared lib.

1. remove general mojo typeExternalWallet and create specific types

2. create new structure in preferences

• new object wallets
• rename wallet into brave and move it in wallets
• deprecate external_wallets
• move external_wallets->uphold in wallets->uphold

3. Create new function GetWallet(string type) which will now return json string and all parsing should be done in shared lib

[LaurenWags]

Marking as QA/Blocked and questions sent to @NejcZdovc for confirmation.

[LaurenWags]

Verified passed with

Brave | 1.16.50 Chromium: 86.0.4240.55 (Official Build) dev (x86_64)
-- | --
Revision | a6d625ef6f7fe8ea0675f1cf759155a05ee1be40-refs/branch-heads/4240@{#953}
OS | macOS Version 10.14.6 (Build 18G3020)

Verified test plan from brave/brave-core#6627

On 1.15.x enabled Rewards and connected Uphold account. This is from my Preferences file:

Upgraded to 1.16.x and confirmed expected items in Preferences file:

---

Verification passed on

Brave	1.16.50 Chromium: 86.0.4240.55 (Official Build) dev (x86_64)
Revision	a6d625ef6f7fe8ea0675f1cf759155a05ee1be40-refs/branch-heads/4240@{#953}
OS	Ubuntu 18.04 LTS

Verified test plan from brave/brave-core#6627

On 1.15.x enabled Rewards and connected Uphold account. This is from my Preferences file:

Upgraded to 1.16.x and confirmed expected items in Preferences file:

---

Verification passed on

Brave | 1.16.58 Chromium: 86.0.4240.75 (Official Build) dev (64-bit)
-- | --
Revision | c69c33933bfc72a159aceb4aeca939eb0087416c-refs/branch-heads/4240@{#1149}
OS | Windows 10 OS Version 1903 (Build 18362.1082)

Verified test plan from brave/brave-core#6627

On 1.15.x enabled Rewards and connected Uphold account. This is from my Preferences file:

Upgraded to 1.16.x and confirmed expected items in Preferences file: