Chore: Refactor usage of legacy data contracts (#41218)

Refactor usage of legacy data contracts. Moves legacy data contracts 
to pkg/tsdb/legacydata package.
Refactor pkg/expr to be a proper service/dependency that can be provided 
to wire to remove some unneeded dependencies to SSE in ngalert and other places.
Refactor pkg/expr to not use the legacydata,RequestHandler and use 
backend.QueryDataHandler instead.

pkg/setting/setting.go

@@ -203,6 +203,7 @@ type Cfg struct {
 	EnforceDomain    bool
 
 	// Security settings
+	SecretKey             string
 	EmailCodeValidMinutes int
 
 	// build
@@ -1139,6 +1140,7 @@ func (cfg *Cfg) SectionWithEnvOverrides(s string) *DynamicSection {
 func readSecuritySettings(iniFile *ini.File, cfg *Cfg) error {
 	security := iniFile.Section("security")
 	SecretKey = valueAsString(security, "secret_key", "")
+	cfg.SecretKey = SecretKey
 	DisableGravatar = security.Key("disable_gravatar").MustBool(true)
 	cfg.DisableBruteForceLoginProtection = security.Key("disable_brute_force_login_protection").MustBool(false)