feat: add expires_in API with backward compatibility for BTL

[m00npl]

Summary

This PR introduces a new, more intuitive API for managing entity expiration times while maintaining full backward compatibility with the existing BTL (Block-to-Live) system.

Changes

New ExpirationTime Builder Class

• Helper class for time-to-blocks conversion
• Class methods: from_seconds(), from_hours(), from_days(), from_blocks()
• Automatic conversion: 1 block = 2 seconds

New API Fields

• expires_in (int | ExpirationTime) replaces btl in ArkivCreate and ArkivUpdate
• duration (int | ExpirationTime) replaces number_of_blocks in ArkivExtend
• When using int, it represents duration in seconds (not blocks)

Examples

Simple way - seconds as int:

create = ArkivCreate(
    data=b"Hello",
    expires_in=3600,  # 1 hour in seconds
    string_annotations=[],
    numeric_annotations=[]
)

With builder - more readable:

create = ArkivCreate(
    data=b"Hello",
    expires_in=ExpirationTime.from_hours(24),
    string_annotations=[],
    numeric_annotations=[]
)

Legacy API (deprecated but still works):

create = ArkivCreate(
    data=b"Hello",
    btl=1800,  # blocks - shows deprecation warning
    string_annotations=[],
    numeric_annotations=[]
)

Backward Compatibility

Deprecated but Functional

• btl field (in ArkivCreate and ArkivUpdate)
• number_of_blocks field (in ArkivExtend)

Priority System

• expires_in > btl
• duration > number_of_blocks
• Deprecation warnings displayed when using old API

Migration

• Existing code continues to work without changes
• Console warnings encourage migration to new API
• No breaking changes

Technical Details

• Added resolve_expiration_blocks() for expires_in/btl resolution
• Added resolve_extension_blocks() for duration/number_of_blocks resolution
• Updated utils.py to use resolution functions in RLP encoding
• All conversions handle both int (seconds) and ExpirationTime objects

Migration Guide

Users can migrate gradually:

# Before (still works)
ArkivCreate(data=b"Hello", btl=1800, ...)

# After - Option 1: Direct seconds
ArkivCreate(data=b"Hello", expires_in=3600, ...)

# After - Option 2: With builder
ArkivCreate(data=b"Hello", expires_in=ExpirationTime.from_hours(1), ...)