feat: parse SSE streaming responses for response plugins

[Mohammad-nassar10]

What type of PR is this?
/kind bug

What this PR does / why we need it:
Clone the PR #138 from main.

[davidbreitgand]

@Mohammad-nassar10 , @ronenkat , PR looks good, but there is a subtle bug that I spotted. Left a suggestion (validated locally on my side) how to fix.

IN SHORT:
The exact faulty assumption is that there is only one data, but in SSE it may need to be the concatenation of several consecutive data: lines from the same event before attempting unmarshalling.

You can verify that by adding the attached unit test to response_test.go

// This test locks in the multiline SSE fix by asserting that there is one logical SSE event
// split across multiple `data:` lines is reconstructed before JSON parsing.
func TestParseSSEResponseBody_MultilineEvent(t *testing.T) {
	body := []byte("data: {\"response\":\n" +
		"data: {\"model\":\"gpt-4o\",\"usage\":{\"input_tokens\":12,\"output_tokens\":34}}}\n\n" +
		"data: [DONE]\n")

	got, err := parseSSEResponseBody(body)
	if err != nil {
		t.Fatalf("parseSSEResponseBody returned unexpected error: %v", err)
	}

	want := map[string]any{
		"model": "gpt-4o",
		"usage": map[string]any{
			"input_tokens":  float64(12),
			"output_tokens": float64(34),
		},
	}
	if diff := cmp.Diff(want, got); diff != "" {
		t.Fatalf("parseSSEResponseBody returned unexpected result, diff(-want, +got): %s", diff)
	}
}

This line breaking is permitted in response and actually is often happening for SSE streaming