feat(ui): path prefix support via HTTP header (#4497)

Makes the web app honour the `X-Forwarded-Prefix` HTTP request header that may be sent by a reverse-proxy in order to inform the app that its public routes contain a path prefix.
For instance this allows to serve the webapp via a reverse-proxy/ingress controller under a path prefix/sub path such as e.g. `/localai/` while still being able to use the regular LocalAI routes/paths without prefix when directly connecting to the LocalAI server.

Changes:
* Add new `StripPathPrefix` middleware to strip the path prefix (provided with the `X-Forwarded-Prefix` HTTP request header) from the request path prior to matching the HTTP route.
* Add a `BaseURL` utility function to build the base URL, honouring the `X-Forwarded-Prefix` HTTP request header.
* Generate the derived base URL into the HTML (`head.html` template) as `<base/>` tag.
* Make all webapp-internal URLs (within HTML+JS) relative in order to make the browser resolve them against the `<base/>` URL specified within each HTML page's header.
* Make font URLs within the CSS files relative to the CSS file.
* Generate redirect location URLs using the new `BaseURL` function.
* Use the new `BaseURL` function to generate absolute URLs within gallery JSON responses.

Closes #3095

TL;DR:
The header-based approach allows to move the path prefix configuration concern completely to the reverse-proxy/ingress as opposed to having to align the path prefix configuration between LocalAI, the reverse-proxy and potentially other internal LocalAI clients.
The gofiber swagger handler already supports path prefixes this way, see e2d9e9916d/swagger.go (L79)

Signed-off-by: Max Goltzsche <max.goltzsche@gmail.com>

core/http/app_test.go

@@ -237,6 +237,31 @@ func postInvalidRequest(url string) (error, int) {
 	return nil, resp.StatusCode
 }
 
+func getRequest(url string, header http.Header) (error, int, []byte) {
+
+	req, err := http.NewRequest("GET", url, nil)
+	if err != nil {
+		return err, -1, nil
+	}
+
+	req.Header = header
+
+	client := &http.Client{}
+	resp, err := client.Do(req)
+	if err != nil {
+		return err, -1, nil
+	}
+
+	defer resp.Body.Close()
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return err, -1, nil
+	}
+
+	return nil, resp.StatusCode, body
+}
+
 const bertEmbeddingsURL = `https://gist.githubusercontent.com/mudler/0a080b166b87640e8644b09c2aee6e3b/raw/f0e8c26bb72edc16d9fbafbfd6638072126ff225/bert-embeddings-gallery.yaml`
 
 //go:embed backend-assets/*
@@ -345,6 +370,33 @@ var _ = Describe("API test", func() {
 			})
 		})
 
+		Context("URL routing Tests", func() {
+			It("Should support reverse-proxy when unauthenticated", func() {
+
+				err, sc, body := getRequest("http://127.0.0.1:9090/myprefix/", http.Header{
+					"X-Forwarded-Proto":  {"https"},
+					"X-Forwarded-Host":   {"example.org"},
+					"X-Forwarded-Prefix": {"/myprefix/"},
+				})
+				Expect(err).To(BeNil(), "error")
+				Expect(sc).To(Equal(401), "status code")
+				Expect(string(body)).To(ContainSubstring(`<base href="https://example.org/myprefix/" />`), "body")
+			})
+
+			It("Should support reverse-proxy when authenticated", func() {
+
+				err, sc, body := getRequest("http://127.0.0.1:9090/myprefix/", http.Header{
+					"Authorization":      {bearerKey},
+					"X-Forwarded-Proto":  {"https"},
+					"X-Forwarded-Host":   {"example.org"},
+					"X-Forwarded-Prefix": {"/myprefix/"},
+				})
+				Expect(err).To(BeNil(), "error")
+				Expect(sc).To(Equal(200), "status code")
+				Expect(string(body)).To(ContainSubstring(`<base href="https://example.org/myprefix/" />`), "body")
+			})
+		})
+
 		Context("Applying models", func() {
 
 			It("applies models from a gallery", func() {