Support wallet_watchAsset

[yrliou]

Resolves brave/brave-browser#17878

Image parameter is not supported yet because we currently don't support getting a logo from a remote URL, currently it will still use the placeholder icon if a remote URL is passed. (brave/brave-browser#20000)

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

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, 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-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

1. Visit https://www.coingecko.com/en/coins/railgun and click on the fox icon to add suggest token

2. Prompt should show up for adding suggest token (will be asking to unlock first if wallet is locked)

3. Cancel the request.
4. Open a new tab and visit brave://wallet/crypto/portfolio should not see the new token.
5. Do step 1 again and accept the request this time.
6. Go to brave://wallet/crypto/portfolio should see it being listed in visible assets. (Existing page will need to fresh to see the change)

7. Visit https://www.coingecko.com/en/coins/tether and click on the fox icon to add suggest token
8. Prompt should show up with correct logo.

9. Accept the request.
10. Open a new tab and visit brave://wallet/crypto/portfolio should see tether in the asset list.

[Douglashdaniel]

Front-end looks good!

[yrliou]

linux failed at s3-upload.