Add support for lowercase FileFormat(#1362)

* Added support for lowercase FileFormat

Modified the FileFormat class so that it utilizes EnumMeta value aliases. This allows both "AVRO" and "avro" to map to AVRO.

* Make mypy happy

---------

Co-authored-by: Fokko Driesprong <fokko@apache.org>

Author: hgollakota

pyiceberg/manifest.py

@@ -94,9 +94,16 @@ def __repr__(self) -> str:
 
 
 class FileFormat(str, Enum):
-    AVRO = "AVRO"
-    PARQUET = "PARQUET"
-    ORC = "ORC"
+    AVRO = "AVRO", "avro"
+    PARQUET = "PARQUET", "parquet"
+    ORC = "ORC", "orc"
+
+    def __new__(cls, value: str, *value_aliases: List[str]) -> "FileFormat":
+        obj = str.__new__(cls)
+        obj._value_ = value
+        for alias in value_aliases:
+            cls._value2member_map_[alias] = obj
+        return obj
 
     @classmethod
     def _missing_(cls, value: object) -> Union[None, str]: