feat: Ability to serialize Query to Proto

This PR adds an API to serialize and deserialize Queries to and from RunQueryRequests. The new API endpoints are  and .

google-cloud-firestore/src/main/java/com/google/cloud/firestore/FieldPath.java

@@ -19,6 +19,7 @@
 import com.google.auto.value.AutoValue;
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
+import com.google.firestore.v1.StructuredQuery;
 import java.util.regex.Pattern;
 import javax.annotation.Nonnull;
 
@@ -30,11 +31,13 @@
 @AutoValue
 public abstract class FieldPath extends BasePath<FieldPath> implements Comparable<FieldPath> {
 
+  private static final String DOCUMENT_ID_SENTINEL = "__name__";
+
   /**
    * A special sentinel FieldPath to refer to the ID of a document. It can be used in queries to
    * sort or filter by the document ID.
    */
-  static final FieldPath DOCUMENT_ID = FieldPath.of("__name__");
+  static final FieldPath DOCUMENT_ID = FieldPath.of(DOCUMENT_ID_SENTINEL);
 
   /** Regular expression to verify that dot-separated field paths do not contain ~*[]/. */
   private static final Pattern PROHIBITED_CHARACTERS = Pattern.compile(".*[~*/\\[\\]].*");
@@ -67,6 +70,11 @@ public static FieldPath documentId() {
     return DOCUMENT_ID;
   }
 
+  /** Verifies if the provided path matches the path that is used for the Document ID sentinel. */
+  static boolean isDocumentId(String path) {
+    return DOCUMENT_ID_SENTINEL.equals(path);
+  }
+
   /** Returns a field path from a dot separated string. Does not support escaping. */
   static FieldPath fromDotSeparatedString(String field) {
     if (PROHIBITED_CHARACTERS.matcher(field).matches()) {
@@ -152,4 +160,8 @@ FieldPath createPathWithSegments(ImmutableList<String> segments) {
   public String toString() {
     return getEncodedPath();
   }
+
+  StructuredQuery.FieldReference toProto() {
+    return StructuredQuery.FieldReference.newBuilder().setFieldPath(getEncodedPath()).build();
+  }
 }