[swift] date formatter add support for ISO8601 with and without milliseconds

Created by: 4brunu

This PR allows the Swift4 generator to parse dates with the format ISO8601 with and without milliseconds.

It introduced a breaking change, by deprecating Configuration.dateFormat which is replaced by CodableHelper.dateFormatter, which means that now when someone want's a custom date formatter, instead of providing a string, they will have to provide a DateFormatter which gives more flexibility. If someone is changing Configuration.dateFormat they will receive a compilation error that informs that they should now use CodableHelper.dateFormatter instead. If someone is using the default Configuration.dateFormat value, this will be transparent.

This PR fixes https://github.com/OpenAPITools/openapi-generator/issues/699 and it's a continuation of https://github.com/OpenAPITools/openapi-generator/pull/1454.

PR checklist

• Read the contribution guidelines.
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@jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @4brunu (2019/11)