Prevent synced CPA values to be updated from the API (#30687)

* Prevents the API from updating synced CPA values

The patch functions for CPA values now accept a parameter that checks
if they should allow for synced values to be updated, and prevent
those updates if necessary

* Fix linter

* Fix parameter after merge

---------

Co-authored-by: Miguel de la Cruz <miguel@ctrlz.es>
Co-authored-by: Scott Bishel <scott.bishel@mattermost.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/custom_profile_attributes.go

@@ -133,6 +133,10 @@ type CPAAttrs struct {
 	SAML       string                                                `json:"saml"`
 }
 
+func (c *CPAField) IsSynced() bool {
+	return c.Attrs.LDAP != "" || c.Attrs.SAML != ""
+}
+
 func (c *CPAField) ToPropertyField() *PropertyField {
 	pf := c.PropertyField
 
@@ -234,7 +238,8 @@ func NewCPAFieldFromPropertyField(pf *PropertyField) (*CPAField, error) {
 	}, nil
 }
 
-// SanitizeAndValidatePropertyValue validates and sanitizes the given property value based on the field type
+// SanitizeAndValidatePropertyValue validates and sanitizes the given
+// property value based on the field type
 func SanitizeAndValidatePropertyValue(cpaField *CPAField, rawValue json.RawMessage) (json.RawMessage, error) {
 	fieldType := cpaField.Type