chore: Update sdk_metadata features

[devin-ai-integration]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

N/A - This is a metadata-only change with no code changes requiring tests.

Related issues

None

Describe the solution you've provided

This PR adds the features object to .sdk_metadata.json to document which SDK features the Apex Server SDK supports and when they were introduced.

Features added (in alphabetical order):

• allFlags: 1.0
• experimentation: 1.1
• privateAttrs: 1.0
• track: 1.0
• variationDetail: 1.0
• webProxy: 1.0

The feature versions were sourced from the SDK features documentation table (server-side SDKs A-K section, Apex column) and verified against the CHANGELOG.md. Feature keys use the canonical camelCase names from feature_info.json.

Describe alternatives you've considered

N/A

Additional context

Human review checklist:

• Verify the feature versions match the documentation table for the Apex SDK
• Confirm no supported features were missed from the documentation

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Link to Devin run: https://app.devin.ai/sessions/b24d3a1c4f0a4809a2c0a356f13e19c8
Requested by: @keelerm84

[devin-ai-integration]

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