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/export_test.go

@@ -26,3 +26,8 @@ func InitializeConfigurationsTable(db *sqlx.DB) error {
 func ResolveConfigFilePath(path string) (string, error) {
 	return resolveConfigFilePath(path)
 }
+
+// GetCommonStore exposes the internal commonStore to test only.
+func (ms *memoryStore) MergeConfig(patch *model.Config) (*model.Config, error) {
+	return ms.mergeConfig(patch)
+}