feat(hosts): give a host a pool of SNIs, random per config or shared (#6923)

Rotating SNIs meant one host entry per domain. A host now takes a pool of
SNIs: with more than one, every generated config gets a random entry,
or, with the shared switch on, the whole host group uses one pick per
subscription fetch. Raw, JSON and Clash output, the panel's link preview
and node adoption all carry the pool.

sni stays the pool's first entry, so a rolled-back binary and API callers
that only know the single field keep working; an sni outside the pool
becomes its primary. A host with one SNI renders exactly as before.

internal/web/entity/entity.go

@@ -428,6 +428,8 @@ type HostGroup struct {
 	Port                   int      `json:"port" validate:"gte=0,lte=65535"`
 	Security               string   `json:"security" validate:"omitempty,oneof=same tls none reality"`
 	Sni                    string   `json:"sni"`
+	Snis                   []string `json:"snis"`
+	SniSharedRandom        bool     `json:"sniSharedRandom"`
 	HostHeader             string   `json:"hostHeader"`
 	Path                   string   `json:"path"`
 	Alpn                   []string `json:"alpn"`