Update soruceddl with columna and table getter

Author: Deep1998

v2/spanner-common/src/main/java/com/google/cloud/teleport/v2/spanner/migrations/schema/Schema.java

@@ -89,6 +89,10 @@ public Schema(
     this.spSchema = spSchema;
     this.syntheticPKeys = syntheticPKeys;
     this.srcSchema = srcSchema;
+    this.toSpanner = new HashMap<String, NameAndCols>();
+    this.toSource = new HashMap<String, NameAndCols>();
+    this.srcToID = new HashMap<String, NameAndCols>();
+    this.spannerToID = new HashMap<String, NameAndCols>();
     this.empty = (spSchema == null || srcSchema == null);
   }
 

v2/spanner-common/src/main/java/com/google/cloud/teleport/v2/spanner/sourceddl/SourceSchema.java

@@ -54,4 +54,8 @@ public abstract static class Builder {
 
     public abstract SourceSchema build();
   }
+
+  public SourceTable table(String tableName) {
+    return tables().get(tableName);
+  }
 }

v2/spanner-common/src/main/java/com/google/cloud/teleport/v2/spanner/sourceddl/SourceTable.java

@@ -66,4 +66,13 @@ public abstract static class Builder {
 
     public abstract SourceTable build();
   }
+
+  public SourceColumn column(String name) {
+    for (SourceColumn c : columns()) {
+      if (c.name().equals(name)) {
+        return c;
+      }
+    }
+    return null;
+  }
 }