Describe the bug
In our codebase we are using FieldDefinition.Builder like:

builder.inputValueDefinition(inputValueDefinition);

with current master (2020-11-10T10-26-00-07ca7ee, commit 07ca7ee) we have:

java.lang.UnsupportedOperationException
at graphql.com.google.common.collect.ImmutableCollection.add(ImmutableCollection.java:246)
at graphql.language.FieldDefinition$Builder.inputValueDefinition(FieldDefinition.java:210)

IMHO the cause is that in the copy constructor we have this:

this.inputValueDefinitions = existing.getInputValueDefinitions();

whereas should be like this:

this.inputValueDefinitions = new ArrayList<>(existing.getInputValueDefinitions());

because it is a builder and it is supposed to be mutable.

To Reproduce

Use of FieldDefinition.Builder.