Export constants for data length limitations

[Igor Shadurin]

I propose exporting constants that represent data length limitations for

• filename length
• max pods count
• max directory name length
• max full path length
• pod name length

Rationale:
By exporting these constants, it provides the ability for external applications or services to leverage and adhere to the established limitations set within the platform. This ensures consistency and prevents potential errors that might arise from hardcoded values or assumptions about these limitations.

Related: fairDataSociety/fairdrive-theapp#291