feat: RESP3 notifications support & Hitless notifications handling [CAE-1088] & [CAE-1072] (#3418)

- Adds support for handling push notifications with RESP3. 
- Using this support adds handlers for hitless upgrades.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Hristo Temelski <hristo.temelski@redis.com>

internal/pool/pubsub.go

@@ -0,0 +1,78 @@
+package pool
+
+import (
+	"context"
+	"net"
+	"sync"
+	"sync/atomic"
+)
+
+type PubSubStats struct {
+	Created   uint32
+	Untracked uint32
+	Active    uint32
+}
+
+// PubSubPool manages a pool of PubSub connections.
+type PubSubPool struct {
+	opt       *Options
+	netDialer func(ctx context.Context, network, addr string) (net.Conn, error)
+
+	// Map to track active PubSub connections
+	activeConns sync.Map // map[uint64]*Conn (connID -> conn)
+	closed      atomic.Bool
+	stats       PubSubStats
+}
+
+func NewPubSubPool(opt *Options, netDialer func(ctx context.Context, network, addr string) (net.Conn, error)) *PubSubPool {
+	return &PubSubPool{
+		opt:       opt,
+		netDialer: netDialer,
+	}
+}
+
+func (p *PubSubPool) NewConn(ctx context.Context, network string, addr string, channels []string) (*Conn, error) {
+	if p.closed.Load() {
+		return nil, ErrClosed
+	}
+
+	netConn, err := p.netDialer(ctx, network, addr)
+	if err != nil {
+		return nil, err
+	}
+	cn := NewConnWithBufferSize(netConn, p.opt.ReadBufferSize, p.opt.WriteBufferSize)
+	cn.pubsub = true
+	atomic.AddUint32(&p.stats.Created, 1)
+	return cn, nil
+
+}
+
+func (p *PubSubPool) TrackConn(cn *Conn) {
+	atomic.AddUint32(&p.stats.Active, 1)
+	p.activeConns.Store(cn.GetID(), cn)
+}
+
+func (p *PubSubPool) UntrackConn(cn *Conn) {
+	atomic.AddUint32(&p.stats.Active, ^uint32(0))
+	atomic.AddUint32(&p.stats.Untracked, 1)
+	p.activeConns.Delete(cn.GetID())
+}
+
+func (p *PubSubPool) Close() error {
+	p.closed.Store(true)
+	p.activeConns.Range(func(key, value interface{}) bool {
+		cn := value.(*Conn)
+		_ = cn.Close()
+		return true
+	})
+	return nil
+}
+
+func (p *PubSubPool) Stats() *PubSubStats {
+	// load stats atomically
+	return &PubSubStats{
+		Created:   atomic.LoadUint32(&p.stats.Created),
+		Untracked: atomic.LoadUint32(&p.stats.Untracked),
+		Active:    atomic.LoadUint32(&p.stats.Active),
+	}
+}