iso8601 format not detected correctly

[matejsp]

Date and DateTime with format argument as iso8601 it is not rendered correctly.

Actually marshallow supports also the following:

        "iso": utils.isoformat,
        "iso8601": utils.isoformat,
        "rfc": utils.rfcformat,
        "rfc822": utils.rfcformat,

works:
datetime = fields.DateTime(dump_only=True, format='iso')
does not work:
datetime2 = fields.DateTime(dump_only=True, format='iso8601')

https://github.com/marshmallow-code/marshmallow/blob/dev/src/marshmallow/fields.py#L1275-L1291

[sloria]

We haven't explicitly supported "iso8601" and "rfc822" (the docs only mention iso). But given that they're supported in marshmallow makes sense to support them here, too.

looks like the relevant check is here: https://github.com/marshmallow-code/apispec/blob/9c3ac165bc08af856b13d283c82e838c57919ffa/src/apispec/ext/marshmallow/field_converter.py#L571

PR welcome!