Large refactor - it compiles!

This refactor splits out all conversation concerns into a new
`conversation` package. There is now a split between `conversation` and
`api`s representation of `Message`, the latter storing the minimum
information required for interaction with LLM providers. There is
necessary conversation between the two when making LLM calls.

pkg/provider/provider.go

@@ -0,0 +1,41 @@
+package provider
+
+import (
+	"context"
+
+	"git.mlow.ca/mlow/lmcli/pkg/api"
+)
+
+type Chunk struct {
+	Content    string
+	TokenCount uint
+}
+
+type RequestParameters struct {
+	Model string
+
+	MaxTokens   int
+	Temperature float32
+	TopP        float32
+
+	Toolbox []api.ToolSpec
+}
+
+type ChatCompletionProvider interface {
+	// CreateChatCompletion generates a chat completion response to the
+	// provided messages.
+	CreateChatCompletion(
+		ctx context.Context,
+		params RequestParameters,
+		messages []api.Message,
+	) (*api.Message, error)
+
+	// Like CreateChageCompletion, except the response is streamed via
+	// the output channel.
+	CreateChatCompletionStream(
+		ctx context.Context,
+		params RequestParameters,
+		messages []api.Message,
+		chunks chan<- Chunk,
+	) (*api.Message, error)
+}