Make precedence source ordered

[linglingye001]

Why this PR?

Fix #621

ConfigurationFeatureDefinitionProvider always evaluates the Microsoft schema first, source order is never consulted when the same flag is split across schemas. A Microsoft-schema definition from the first provider beats a .NET-schema definition from every later provider.

Visible Changes

• Tracks which schema last defines each feature by configuration-source order.
• Later sources win across .NET and Microsoft schemas when custom merging is enabled.
• Microsoft schema wins when both schemas occur in the same source.
• Deduplicates cross-schema entries during feature enumeration.
• Leaves default, non-custom behavior unchanged.

Example

When custom merging is enabled:

Provider` 1: Microsoft schema → MyFlag = false
Provider 2: .NET schema → MyFlag = targeting filter

Winning schema map:
MyFlag → .NET

Result: Provider 2 wins and the targeting filter is parsed.