Add client detection

[jhrozek]

• Add client detector interface - Adds a decorator that can be added to the FastAPI handlers and detect the client from a fallback mechanism, by the user-agent, by a specific header or by a matching word in the messages. At the moment, the clients are represented as a simple enum, but in follow-up patches they will be represented by classes that can perform the changes by an interface providing callbacks from the pipeline or other places that need client-specific behaviour.
• Use the client type when streaming the data to the client, not when executing completion - We used to special-case ollama stream generation by passing the client type to the execute_completion. Instead, let's pass the client type to the place that needs special casing using the recently introduce client type enum.
• Use the client type when instantiating and running provider pipelines - Instead of detecting the client type again when the pipeline is being processed, let's pass the client type on instantiating the pipeline instance as a constant and replace the hardcoded client strings by just using the constants.
• Remove get_tool_name_from_messages - This was superseded by using the client enum.
• Remove the is_copilot flag in favor of using the autodetected client - In the copilot provider, we can hardcode the client type to copilot when instantiating the pipelines.

[aponcedeleonch]

Amazing!

[jhrozek]

I rebased the branch and fixed some smallish issues along the way. Ready for a next round.

[jhrozek]

Good guy integration test, catching my bugs.

[lukehinds]

Good work @jhrozek , just needs a rebase.