proposal: mime/multipart: FieldContentDisposition helper

[malta895]

I propose the addition of an helper in the package mime/multipart, with the following signature

func FieldContentDisposition(fieldname string) string

The helper implements the escaping logic seen in the helper CreateFormField, that adds to a part the Content-Disposition header with field name only (no file name), allowing users to avoid reimplementing such logic and to have the possibility to define additional custom Mime Headers, having the Content-Disposition header still generated and escaped by Go standard library.

This proposal is related to #46771, which addresses the helper FileContentDisposition with the file name too.

[ianlancetaylor]

CC @robtimus @neild