Parse Unixtime fields as Date in the `client` package

[mois-ilya]

Implement parsing for all fields with unixtime in the Swagger schema as Date objects in the client package. This will involve introducing either a custom field or a custom format for integers in the schema to represent unixtime fields. Additionally, a custom converter will need to be written to handle this type.

Requirements:

• Add a custom field or agree on a custom format for integers in the Swagger schema to indicate unixtime fields.
• Implement a custom converter in the client package that parses these fields as Date objects.

Acceptance Criteria:

• Unit tests are written to verify the correct parsing of unixtime fields as Date objects.