Display hex string for signing when string is not a UTF8 string

[bbondy]

When the string is printable we display the string, otherwise we will now display the 0x prefixed hex of the signing data.

Resolves brave/brave-browser#19228

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:

Clone this:
https://github.com/bbondy/eth-manual-tests

python -m SimpleHTTPServer 8081
Navigate to http://localhost:8081/request.html

Use eth_requestAccounts so you can eth_sign with the test data below.

Here are 3 test cases to try:

0x68656c6c6f20776f726c64
Should produce: hello world

0xdeadbeef
Should produce 0xdeadbeef

Unicode string
0x4120756e69636f646520c68e20737472696e6720c3b1

[darkdh]

++
Note that QA cannot test this on opensea.io until brave/brave-browser#18659 is closed.
Or we can specify something else in test plan.

[bbondy]

Note that QA cannot test this on opensea.io until brave/brave-browser#18659 is closed.
Or we can specify something else in test plan.

I specified something else in the test plan above 👍