Added support to capture timestamp

[mvanbeusekom]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Feature

⤵️ What is the current behavior?

Currently the returned Position instance doesn't contain a timestamp indicating when the position was determined.

🆕 What is the new behavior (if this is a feature change)?

This PR adds a timestamp to the Position instance

💥 Does this PR introduce a breaking change?

No

🐛 Recommendations for testing

Run the example application (timestamp is shown on the location stream page):

# change into the example folder
cd example
# run the example app
flutter run

📝 Links to relevant issues/docs

• Fixes GPS Timestamp in Position #98

🤔 Checklist before submitting

• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated
• Rebased onto current develop