Switch to Chainstack RPC proxy endpoints

[onyb]

Description

We're currently using Infura as our main RPC provider, and the code in brave-core hardcodes this. We'd like to use a generic proxy URL format, which uses Chainstack under the hood. This allows us to easily switch providers whenever we want.

Steps to reproduce

N/A

Actual result

Portfolio balances should load correctly without any console errors. Infura RPCs are used for making requests.

Expected result

Portfolio balances should load correctly without any console errors. Chainstack RPCs are used for making requests.

Reproduces how often

Easily reproduced

Brave version (brave://version info)

1.66.118

Channel information

• release (stable)
• beta
• nightly

Reproducibility

• with Brave Shields disabled
• with Brave Rewards disabled
• in the latest version of Chrome

Miscellaneous information

Ideally uplift this to 1.67.x so we can save on Infura bills. We're currently paying for both Infura as well as Chainstack RPCs.

[kjozwiak]

The above requires 1.68.92 or higher for 1.68.x verification 👍

[srirambv]

Verification passed on

Brave	1.68.92 Chromium: 126.0.6478.71 (Official Build) beta (64-bit)
Revision	393b9968e233540a48f04d74bc4601a05d3a0169
OS	Windows 11 Version 23H2 (Build 22631.3737)

• Verified steps from brave/brave-core#24045
• Verified balances work as expected on Portfolio
• Verified no console errors when loading Portfolio balance