This PR implements support forExtraHeaderParameters andExtraQueryParameters properties onDownstreamApiOptions to simplify adding custom headers and query parameters to downstream API requests.

Problem

Previously, developers had to use the complexCustomizeHttpRequestMessage delegate to add custom headers and query parameters to downstream API calls:

varresult=awaitdownstreamApi.GetForAppAsync<string>("Api",    options=>{options.CustomizeHttpRequestMessage= requestMessage=>{requestMessage.Headers.Add("OData-Version","4.0");// Query parameters would need manual URL manipulation};});

Solution

This PR adds support for the more convenientExtraHeaderParameters andExtraQueryParameters properties:

varresult=awaitdownstreamApi.GetForAppAsync<string>("Api",    options=>{options.ExtraHeaderParameters=newDictionary<string,string>{{"OData-Version","4.0"},{"X-Custom-Header","value"}};options.ExtraQueryParameters=newDictionary<string,string>{{"version","v2"},{"format","json"}};});

Implementation Details

Key Features

• Reflection-based detection - Uses reflection to check if properties exist onDownstreamApiOptions for backward compatibility
• Dual naming support - Supports both "ExtraHeaderParameters" and "ExtraHeadersParameters" naming conventions
• Proper URL encoding - Query parameters are automatically URL-encoded usingUri.EscapeDataString
• Maintains existing behavior - Works alongside existingCustomizeHttpRequestMessage delegate
• Comprehensive test coverage - Tests for headers, query parameters, edge cases, and special characters

Technical Changes

• AddedGetExtraHeaderParameters() andGetExtraQueryParameters() helper methods inDownstreamApi class
• EnhancedUpdateRequestAsync() method to apply extra parameters beforeCustomizeHttpRequestMessage is called
• Updated all target framework InternalAPI files with new method signatures
• Created comprehensive test suite inExtraParametersTests.cs

Header Processing

• Headers are added usingTryAddWithoutValidation to support custom header schemes
• Applied beforeCustomizeHttpRequestMessage delegate, allowing for override
• Empty or null dictionaries are safely ignored

Query Parameter Processing

• Parameters are properly URL-encoded for safety
• Correctly appends to existing query parameters in the URL
• Handles special characters and spaces correctly

Testing

The implementation includes comprehensive tests covering:

• Adding extra headers to requests
• Adding extra query parameters with proper URL encoding
• Appending to existing query parameters
• Handling special characters and spaces
• Edge cases with null/empty dictionaries
• Alternate naming conventions (ExtraHeadersParameters vs ExtraHeaderParameters)

Backward Compatibility

The implementation uses reflection to detect if these properties exist onDownstreamApiOptions, ensuring full backward compatibility with different versions of the Microsoft.Identity.Abstractions package.

Dependencies

This feature assumes that theExtraHeaderParameters andExtraQueryParameters properties will be added toDownstreamApiOptions in the Microsoft.Identity.Abstractions package as mentioned in the related issue.

Fixes#3387.

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