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

proxy/proxymanager.go

@@ -151,7 +151,7 @@ func New(proxyConfig config.Config) *ProxyManager {
 		muxLogger:      muxLogger,
 		upstreamLogger: upstreamLogger,
 
-		metricsMonitor: newMetricsMonitor(proxyLogger, maxMetrics),
+		metricsMonitor: newMetricsMonitor(proxyLogger, maxMetrics, proxyConfig.CaptureBuffer),
 
 		processGroups: make(map[string]*ProcessGroup),