feat: Add read-only getters for `Point.fields` and `Point.tags`

[g3n35i5]

This makes testing easier.

Proposed Changes

I am currently contributing to a Jenkins Plugin which uses InfluxDB. Testing this plugin currently requires the use of Java reflections in order to compare written tags and fields.

Adding read-only getter functions for tags and fields would make this more easy.

Checklist

• [x] CHANGELOG.md updated
• [X] Rebased/mergeable
• [X] A test has been added if appropriate
• [X] mvn test completes successfully
• [X] Commit messages are conventional
• [X] Sign CLA (if not already signed)