[MM-30436] Add support for configuration custom defaults (#16265)

* [MM-30436] Add support for configuration custom defaults

* Addressing review comments

* Addressing PR comments

* Soft fail if the configuration defaults cannot be read

* Directly print error in log message

* Fix linter

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

config/common_test.go

@@ -14,7 +14,7 @@ import (
 	"github.com/mattermost/mattermost-server/v5/model"
 )
 
-var emptyConfig, readOnlyConfig, minimalConfig, invalidConfig, fixesRequiredConfig, ldapConfig, testConfig *model.Config
+var emptyConfig, readOnlyConfig, minimalConfig, invalidConfig, fixesRequiredConfig, ldapConfig, testConfig, customConfigDefaults *model.Config
 
 func init() {
 	emptyConfig = &model.Config{}
@@ -72,6 +72,14 @@ func init() {
 			SiteURL: sToP("http://TestStoreNew"),
 		},
 	}
+	customConfigDefaults = &model.Config{
+		ServiceSettings: model.ServiceSettings{
+			SiteURL: model.NewString("http://custom.com"),
+		},
+		DisplaySettings: model.DisplaySettings{
+			ExperimentalTimezone: model.NewBool(false),
+		},
+	}
 }
 
 func TestMergeConfigs(t *testing.T) {
@@ -132,7 +140,7 @@ func TestMergeConfigs(t *testing.T) {
 func TestConfigEnvironmentOverrides(t *testing.T) {
 	memstore, err := config.NewMemoryStore()
 	require.NoError(t, err)
-	base, err := config.NewStoreFromBacking(memstore)
+	base, err := config.NewStoreFromBacking(memstore, nil)
 	require.NoError(t, err)
 	originalConfig := &model.Config{}
 	originalConfig.ServiceSettings.SiteURL = newString("http://notoverriden.ca")
@@ -161,7 +169,7 @@ func TestRemoveEnvironmentOverrides(t *testing.T) {
 
 	memstore, err := config.NewMemoryStore()
 	require.NoError(t, err)
-	base, err := config.NewStoreFromBacking(memstore)
+	base, err := config.NewStoreFromBacking(memstore, nil)
 	require.NoError(t, err)
 	oldCfg := base.Get()
 	assert.Equal(t, "http://overridden.ca", *oldCfg.ServiceSettings.SiteURL)