MM-14442: Merge function, to enable merging of configs (#10423)

* MM-14442: Merge function, to enable merging of configs

Merge will return a new struct of the same type as base and
patch, with patch merged into base. Specifically, patch's values will be preferred
except when patch's value is `nil`.

Restrictions/guarantees:
- base or patch will not be modified
- base and patch can be pointers or values
- base and patch must be the same type
- base and patch must have no unexported types (at the moment)
- if maps or slices are different, the entire map or slice will be
  replaced (at the moment)

* License header

* major rewrite addressing PR comments from Jesse

* MM-14442: merge function for config files

- simplified merge for slices and maps
- fixed many problems with nested pointers/maps/slices
- all references are cloned
- 54 new tests, simplified tests for specific problems

* MM-14442: fixing formatting, comments

config/common.go

@@ -4,6 +4,7 @@
 package config
 
 import (
+	"github.com/mattermost/mattermost-server/utils"
 	"io"
 	"sync"
 
@@ -140,3 +141,14 @@ func (cs *commonStore) validate(cfg *model.Config) error {
 
 	return nil
 }
+
+// mergeConfig merges two configs together. The receiver's values are overwritten with the patch's
+// values except when the patch's values are nil.
+func (cs *commonStore) mergeConfig(patch *model.Config) (*model.Config, error) {
+	ret, err := utils.Merge(cs.config, patch)
+	if err != nil {
+		return nil, err
+	}
+	retC := ret.(model.Config)
+	return &retC, nil
+}