Graphql field enums

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What does this MR do and why?

This MR aims to expand the use of GRAPQL field enums while reducing the use of plain strings.

Reference issue: #16

src/analyzer/field.rs

@@ -22,12 +22,12 @@ impl Field {
         }
     }
 
-    pub fn validate_field(&self) -> Result<(), GlqlError> {
+    pub fn validate_field(&self) -> Result<&Field, GlqlError> {
         match self {
             Unknown(f) => Err(GlqlError::analyzer_error(UnrecognizedField {
                 field: f.to_string(),
             })),
-            _ => Ok(()),
+            _ => Ok(self),
         }
     }