feat(capture): add a signature with timestamp parameter

[ioannisj]

💡 Motivation and Context

Add option to pass a custom timestamp when calling capture()

see: #210

💚 How did you test it?

• Unit test
• Updated Objc project to test for public API changes

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.