Add Dax.Metadata.Calendar, bump model version to 1.9.0

[albertospelta]

This pull request introduces support for Calendar feature, enabling tables to have associated calendar metadata and supporting advanced time-based grouping and associations. The implementation includes new domain classes, JSON serialization/deserialization logic, and integration into the model extraction process.

New Calendar feature and model integration:

• Added new Calendar domain model (Calendar, CalendarColumnGroup, TimeRelatedColumnGroup, TimeUnitColumnAssociation) to represent calendar metadata, including time-based column groups and associations. (src/Dax.Metadata/Calendar.cs, src/Dax.Metadata/CalendarColumnGroup.cs, [1] [2]
• Introduced TimeUnit enum to support various time units (year, quarter, month, etc.) for calendar associations. (src/Dax.Metadata/TimeUnit.cs, src/Dax.Metadata/TimeUnit.csR1-R31)

Serialization and deserialization improvements:

• Implemented a custom JSON converter (CalendarColumnGroupCustomCreationConverter) to correctly deserialize calendar column group types based on their properties. (src/Dax.Metadata/JsonConverters/CalendarColumnGroupCustomCreationConverter.cs, src/Dax.Metadata/JsonConverters/CalendarColumnGroupCustomCreationConverter.csR1-R42)

Model and extractor integration:

• Updated the Table class to support multiple calendars, including serialization hooks to maintain references. (src/Dax.Metadata/Table.cs, [1] [2] [3]
• Enhanced the TOM extractor to extract calendar metadata from TOM tables, including column group and time unit associations. (src/Dax.Model.Extractor/TomExtractor.cs, [1] [2]

Versioning:

• Incremented the model version to 1.9.0 to reflect the new calendar feature. (src/Dax.Metadata/Model.cs, src/Dax.Metadata/Model.csL114-R114)