Faster serialization of Avro unions

[piotrp]

Describe your changes

Checklist before merge

• [ ] Related issue ID is placed at the beginning of PR title in [brackets] (can be GH issue or Nu Jira issue)
• [x] Code is cleaned from temporary changes and commented out lines
• [x] Parts of the code that are not easy to understand are documented in the code
• [ ] Changes are covered by automated tests
• [ ] Showcase in dev-application.conf added to demonstrate the feature
• [ ] Documentation added or updated
• [x] Added entry in Changelog.md describing the change from the perspective of a public distribution user
• [ ] Added MigrationGuide.md entry in the appropriate subcategory if introducing a breaking change
• [ ] Verify that PR will be squashed during merge

[github-actions[bot]]

created: #6882 :warning: Be careful! Snapshot changes are not necessarily the cause of the error. Check the logs.

[lciolecki]

What about encodeRecord? There is also $schema in error.

[piotrp]

I left full schema in places that are likely to be called directly, not as part of a union. In case of records it may be helpful to have a full schema in error message.

This PR is meant as a quick fix to work around the most important inefficiency: handling schemas with nulls (e.g. type: ["null", "string"]). This entire encoder should be rewritten to handle unions efficiently.