feat(upstream): Add upstream proxy/cache subsystem and integrate per-protocol upstreams

ts/upstream/index.ts

@@ -0,0 +1,11 @@
+// Interfaces and types
+export * from './interfaces.upstream.js';
+
+// Classes
+export { CircuitBreaker, CircuitOpenError, withCircuitBreaker } from './classes.circuitbreaker.js';
+export type { ICircuitBreakerMetrics } from './classes.circuitbreaker.js';
+
+export { UpstreamCache } from './classes.upstreamcache.js';
+export type { ICacheSetOptions, ICacheStats } from './classes.upstreamcache.js';
+
+export { BaseUpstream } from './classes.baseupstream.js';