proxy,ui-svelte: add request/response capturing (#508)

Add saving request and response headers and bodies that go through
llama-swap in memory.

- captureBuffer added to configuration. Captures are enabled by default.
- 5MB of memory is allocated for req/response captures in a ring buffer.
Setting captureBuffer to 0 will disable captures.
- UI elements to view captured data added to Activity page. Includes
some
QOL features like json formatting and recombining SSE chat streams
- capture saving is done at the byte level and has minimal impact on
llama-swap performance

Fixes #464 
Ref #503

ui-svelte/src/lib/types.ts

@@ -21,6 +21,16 @@ export interface Metrics {
   prompt_per_second: number;
   tokens_per_second: number;
   duration_ms: number;
+  has_capture: boolean;
+}
+
+export interface ReqRespCapture {
+  id: number;
+  req_path: string;
+  req_headers: Record<string, string>;
+  req_body: string; // base64 encoded bytes
+  resp_headers: Record<string, string>;
+  resp_body: string; // base64 encoded bytes
 }
 
 export interface LogData {