pricesrpc: add additional request context to GetPriceRequest

[Roasbeef]

Right now we only pass along the path of the request to the backend: https://github.com/lightninglabs/aperture/blob/master/pricesrpc/rpc.proto#L12

This is useful or pricing distinct calls to diff paths, but isn't as useful for more advanced use cases. One example is proxying access to an LLM, but scaling the pricing per query, based on things like the chosen model, the context size, etc, etc. All information that will be sent along with the rest as JSON encoded body params.

We should pass along all/some of the other request level context: https://pkg.go.dev/net/http#Request

[Roasbeef]

Relevant context:

aperture/proxy/proxy.go

Lines 158 to 161 in 9b85f8b

	price, err := target.pricer.GetPrice(
	r.Context(), r.URL.Path,
	)
	if err != nil {