public CodecInteraction accessVarDataLength(final Token token)
            {
                return accessVarDataLengthInteractions.computeIfAbsent(
                    token, (t) -> new AccessVarDataLength(groupPathsByField.get(t), t));
            }
Find or create a {@link CodecInteraction} to represent accessing the length
of a variable-length data field without advancing the codec position.

<p>For decoders and codecs, this will be when the length is accessed, e.g.,
{@code decoder.myVarDataLength()}.

<p>The supplied token must carry a {@link Signal#BEGIN_VAR_DATA} signal.

@param token the token identifying the field
@return the {@link CodecInteraction} instance
sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/common/FieldPrecedenceModel.java