Introduce a new JSON extension type in vortex-json

[AdamGS]

Summary

This PR introduces a new JSON extension type, with more work building on top of it planned in the future. This is a building block towards the variant-based compressor and similar work.

API Changes

Includes a minor change, unifiying the two EmptyMetadata types we currently have into one.

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚡ 1 improved benchmark
❌ 2 regressed benchmarks
✅ 1272 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	chunked_varbinview_canonical_into[(1000, 10)]	161.3 µs	197.7 µs	-18.38%
❌	Simulation	bitwise_not_vortex_buffer_mut[128]	246.1 ns	275.3 ns	-10.6%
⚡	Simulation	chunked_varbinview_canonical_into[(100, 100)]	308.3 µs	273.3 µs	+12.8%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing adamg/vortex-json (bd02756) with develop (1552135)}