Add `base::expected<>`-style API to the "fetch balance" flow

[Miyayes]

Most important is balance related.

[Miyayes]

@szilardszaloki Would you be able to rename this issue to something more specific, now that you made some definite changes? :)

[kjozwiak]

The above requires 1.50.94 or higher for 1.50.x verification 👍

[LaurenWags]

Verified with

Brave | 1.50.102 Chromium: 111.0.5563.116 (Official Build) beta (x86_64)
-- | --
Revision | ade86b9472831e26d87fd569e04b404be01e1e2b-refs/branch-heads/5563@{#1209}
OS | macOS Version 12.6.3 (Build 21G419)

• Verified the test plan from https://github.com/brave/brave-core/pull/17480#issue-1610700367

1. Before enabling rewards - PASSED

- Confirmed that there is no `Balance info` displayed on the brave://rewards-internals page before enabling the rewards

2. After enabling rewards - PASSED

- Confirmed that after enabling the rewards the vBAT balance is shown on the Rewards Internals page - Confirmed that the `Connect account` is shown on the brave://rewards settings page, BR panel and rewards NTP widget Example | Example | Example | Example ---- | ---- | ---- | ---- | | |

3. After connecting a custodial wallet - PASSED

- Confirmed that after connecting custodian wallet, both the vBAT/external wallet balance is displayed on the Rewards Internals page - Confirmed that after connecting custodian wallet, total balance is displayed on the Rewards page, Rewards panel and Rewards NTP widget Example | Example | Example | Example ---- | ---- | ---- | ---- | | |

4. Logged out state - PASSED

- Confirmed that in the logged out state,the vBAT balanceis shown only on the Rewards Internals page - Confirmed that `Logged out UI` is shown on the brave://rewards, BR panel, Rewards NTP and Tipping banner - Confirmed that `: ( You are currently logged out` text message is shown on the tipping banner Example | Example | Example | Example | Example ---- | ---- | ---- | ---- | ---- | | | |

5. Fail the fetch balance request - PASSED

- Uphold wallet (pre-req: have Charles Proxy set up to return 500 error for the endpoint (https://api-sandbox.uphold.com/v0/me/cards/*) - Confirmed that spinner animation is shown for both the Rewards BAT (vBAT) and external wallet balance on brave://rewards-internals page Example | Example | Example | Example ---- | ---- | ---- | ---- | | |

6. Reset rewards - PASSED

- Confirmed that after `Reset rewards` the wallet balance info is not displayed on the brave://rewards-internals page

[GeetaSarvadnya]

Verification PASSED on Samsung Galaxy version 12 running Bravemonoarm64.apk_1.50.101

• Verified the test plan from https://github.com/brave/brave-core/pull/17480#issue-1610700367

1. Before enabling rewards - PASSED

- Confirmed that there is no `Balance info` displayed on the brave://rewards-internals page before enabling the rewards 

2. After enabling rewards - PASSED

- Confirmed that after enabling the rewards the vBAT balance is shown on the Rewards Internals page - Confirmed that the `Connect account` is shown on the brave://rewards settings page, BR panel and rewards NTP widget (NA) Example | Example | Example ---- | ---- | ----  |  | 

3. After connecting a custodial wallet - PASSED

- Confirmed that after connecting custodian wallet, both the vBAT/external wallet balance is displayed on the Rewards Internals page - Confirmed that after connecting custodian wallet, total balance is displayed on the Rewards page, Rewards panel and Rewards NTP widget (NA) Example | Example | Example ---- | ---- | ----  |  | 

4. Logged out state - logged #29361

- Confirmed that in the logged out state,the vBAT balanceis shown only on the Rewards Internals page - Confirmed that `Logged out UI` is shown on the brave://rewards, BR panel, Rewards NTP (NA) and Tipping banner - Confirmed that `: ( You are currently logged out` text message is shown on the tipping banner, logged #29361 Example | Example | Example | Example ---- | ---- | ---- | ----  |  |  | 

5. Fail the fetch balance request - PASSED

- Uphold wallet (pre-req: have Charles Proxy set up to return 500 error for the endpoint (https://api-sandbox.uphold.com/v0/me/cards/*) - Confirmed that spinner animation is shown for both the Rewards BAT (vBAT) and external wallet balance on brave://rewards-internals page, encountered #29054 Example | Example | Example ---- | ---- | ----  |  | 

6. Reset rewards - PASSED

- Confirmed that after `Reset rewards` the wallet balance info is not displayed on the brave://rewards-internals page Example | Example ---- | ----  |