feat: add support for reading linked RNTuple attribute sets

[aashirvad08]

This PR adds support for reading linked attribute sets from RNTuples

RNTuple v1.0.1.0 introduced attribute sets as separate linked RNTuples. Uproot was already reading the corresponding footer records but it was not following the locators to actually load the attribute data

And PR exposes them through RNTuple.attributes and adds RNTupleAttributeSet for accessing the user data, ranges and entry-based lookups

It also handles linked RNTuple cache isolation and lazy loading so unsupported attribute schemas only fail when that particular attribute set is accessed

limitation: test_ROOT has not been run because it requires ROOT 6.40 which introduced CreateAttributeSet

closes - #1721

[codecov]

Codecov Report

❌ Patch coverage is 98.80952% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.56%. Comparing base (694b678) to head (86184fc).

Files with missing lines	Patch %	Lines
src/uproot/models/RNTuple.py	98.64%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/uproot/behaviors/RNTuple.py	77.36% <100.00%> (+2.59%)	⬆️
src/uproot/const.py	100.00% <100.00%> (ø)
src/uproot/models/RNTuple.py	91.46% <98.64%> (+0.93%)	⬆️