[Internal] Example: Add unit tests to show DateOnly custom serializer examples

[changanhan]

Pull Request Template

Description

This PR adds a unit test to show an example of DateOnly custom serializer. Customers code is required to make similar changes to properly serialize and deserialize DateOnly object.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• [] New feature (non-breaking change which adds functionality)
• [] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [] This change requires a documentation update

Closing issues

To automatically close an issue: closes #4041

[github-actions]

Please follow the required format: "[Internal] Category: (Adds|Fixes|Refactors|Removes) Description"

Internal should be used for PRs that have no customer impact. This flag is used to help generate the changelog to know which PRs should be included. Examples:
Diagnostics: Adds GetElapsedClientLatency to CosmosDiagnostics
PartitionKey: Fixes null reference when using default(PartitionKey)
[v4] Client Encryption: Refactors code to external project
[Internal] Query: Adds code generator for CosmosNumbers for easy additions in the future.

[kirankumarkolli]

Inactive for more than a quarter and closing