confighelper: add getchoice method

This getter provides a pattern for configuring an item that
within a range of choices.

Signed-off-by:  Eric Callahan <arksine@gmail.com>

moonraker/confighelper.py

@@ -249,6 +249,31 @@ class ConfigHelper:
             self.config.getfloat, option, default,
             above, below, minval, maxval, deprecate)
 
+    def getchoice(
+        self,
+        option: str,
+        choices: Union[Dict[str, _T], List[_T]],
+        default_key: Union[Sentinel, str] = Sentinel.MISSING,
+        force_lowercase: bool = False,
+        deprecate: bool = False
+    ) -> _T:
+        result: str = self._get_option(
+            self.config.get, option, default_key, deprecate=deprecate
+        )
+        if force_lowercase:
+            result = result.lower()
+        if result not in choices:
+            items = list(choices.keys()) if isinstance(choices, dict) else choices
+            raise ConfigError(
+                f"Section [{self.section}], Option '{option}: Value "
+                f"{result} is not a vailid choice.  Must be one of the "
+                f"following {items}"
+            )
+        if isinstance(choices, dict):
+            return choices[result]
+        else:
+            return result  # type: ignore
+
     def getlists(self,
                  option: str,
                  default: Union[Sentinel, _T] = Sentinel.MISSING,