Using PKCE flow for Gemini Authentication, adding shared oauth utility

[ryanml]

Fixes brave/brave-browser#11043

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone
• Public documentation has been updated as necessary. For instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-compatibility-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

General authentication regression for Binance and Gemini

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated.

[ryanml]

We can use this module for more code sharing utilities

[ryanml]

This works for Binance, but Gemini expects characters preserved so these steps are optional

[emerick]

LGTM

[kjozwiak]

Verification PASSED on macOS 10.15.6 x64 using the following build:

Brave | 1.14.44 Chromium: 85.0.4183.69 (Official Build) nightly (64-bit)
-- | --
Revision | 4554ea1a1171bd8d06951a4b7d9336afe6c59967-refs/branch-heads/4183@{#1426}
OS | macOS Version 10.15.6 (Build 19G73)

• Went through a general regression pass for Gemini authentication via Updates Oauth scopes for Gemini #6280 (comment)

Went through the following cases via the Binance widget as per #6306 (comment):

• ensured that you can connect without any issues and the Summary, Deposit, Convert and Buy tabs are displayed
• ensured that disconnecting/reconnecting via the widget worked as expected and displayed the correct blanaces
• ensured that deposit addresses are being displayed correctly under the Deposit tab
• ensured that clicking on the QR code images under the Deposit tab correctly enlarges/displays the QR codes
• ensured that tokens such as HBAR display both a deposit address and memo address
• ensured that selecting Learn More under the kabob/settings menu opened https://brave.com/binance
• ensured that selecting tokens via the Buy tab correctly updates the Buy [Token] button
• ensured that clicking on the Buy button under the Buy tab correctly opens a new tab with the correct amounts already filled out