Support ParquetTimestampResolution.Microseconds for Unity (.Net Standard 2.1)

[spebern]

Issue description

Hello,

We are trying to use this package in Unity and would like to serialize our timestamps with a microsecond resolution. However, Unity only supports .Net Standard 2.1, so ParquetTimestampResolution.Microseconds is not available.

Is there any way to map a long to a parquet timestamp with microsecond resolution?

We were thinking about something like:

[ParquetTimestamp(ParquetTimestampResolution.Microseconds)]
public long TimestampMicros { get; set; }

Thanks in advance, and thank you for this fantastic package!