fix(v1.1.6, #102): switch prime sign secret encoding to utf

[tiagosiebler]

Summary

• Fixes Invalid character error in CBPrimeClient #102

Additional Information

PR Checklist

As part of the PR, make sure you have:

• No breaking changes / documented all breaking changes clearly.
• Updated & checked that all tests pass.
• Increased the version number in the package.json
• Checked npm install runs without issue.
• Included the package-lock.json, if it changed after npm install
• Checked npm run build runs without issue.
• Updated the endpoint map (optional, if you know how).
• Run llms.txt generator (optional, if you know how).