.Net: ADR for function call content model classes (#5696)

Today, in SK, LLM function calling is supported exclusively by the
OpenAI connector, and the function calling model is specific to that
connector. The new AI connectors being added to SK, which support
function calling, introduce their specific models for function calling.
The design, where each new connector introduces its own specific model
class for function calling, does not scale well from the connector
development perspective and does not allow for polymorphic use of
connectors by the SK consumer code.

This ADR describes the high-level details of the service-agnostic
function-calling model classes, while leaving the low-level details to
the implementation phase. Additionally, this ADR outlines the identified
options for various aspects of the design.

Requirements - #5153

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Co-authored-by: Eduard van Valkenburg <eavanvalkenburg@users.noreply.github.com>