.NET: Adding feature collections ADR

[westey-m]

Motivation and Context

Recreating the previous PR #2503 to target the main branch.

Description

• Adding feature collections ADR

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This pull request adds an Architectural Decision Record (ADR) for feature collections and includes references to four other new ADR documents in the solution file. The main ADR (0014-feature-collections.md) documents the decision to use AdditionalProperties dictionaries instead of a dedicated feature collection implementation for passing arbitrary services or data to agents and components in the agent execution stack.

Changes:

• Added ADR 0014 documenting the feature collections design decision and rationale
• Added references to ADRs 0010-0014 in the .NET solution file for visibility
• Included supporting ADRs for Python-specific decisions (get_response simplification and TypedDict options)

Reviewed changes

Copilot reviewed 2 out of 4 changed files in this pull request and generated 9 comments.

File	Description
dotnet/agent-framework-dotnet.slnx	Added references to five new ADR files (0010-0014) in the solution file for better visibility and navigation
docs/decisions/0014-feature-collections.md	New ADR documenting the decision to use AdditionalProperties for feature collections, with detailed analysis of alternatives and sample scenarios
docs/decisions/0013-python-get-response-simplification.md	New ADR documenting the Python-specific decision to consolidate streaming and non-streaming methods into a single get_response method
docs/decisions/0012-python-typeddict-options.md	New ADR documenting the use of TypedDict with generic type parameters for type-safe chat client options in Python