Add Secret utility and rename join method to joinToURL

• Please refer the README to learn more about the Secret utility.
• The join helper was never documented, so I take the lenience to rename it without the breaking change.

Commits

• feat: add Secret utility 7194dd9
• refactor: document and tighten up assert utilities 5e5767c
• refactor: rename join utility to joinToURL 6f3f226
• chore: update dependencies e35104b

Full Changelog: v6.6.0...v6.7.0