bug: update GaxiosError code to include a string or a number

[sofisl]

We need to allow either string or number representation for GaxiosError code.

See this PR describing the problem: https://github.com/googleapis/google-auth-library-nodejs/pull/1165

This is the original bug: https://github.com/googleapis/google-auth-library-nodejs/issues/1092

This will be a breaking change, so we need to wait until the next major version to make this change.

[bcoe]

Making p3, since our intention is to drop Node 10 in 6 months.

[sofisl]

Relabeling as a FR since it's not breaking anyone's functionality currently, but will be a bug we correct in the next version.

[danielbankhead]

Fixed in https://github.com/googleapis/gaxios/pull/552