mime/multipart: Allow limiting maximum amount of part's header data

[andrius4669]

This is feature request, to allow user of mime/multipart package's Reader to specify limit of header data loaded into memory when calling NewPart() function.
Reader.NewPart() calls net/textproto package's Reader.ReadMIMEHeader() function which without any limitation loads all header data into memory.
mime/multipart's Reader is also used in net/http package to parse multipart/form-data POST requests, which means that functions like Request.ParseMultipartForm, Request.FormValue, Request.FormFile, Request.MultipartReader are affected by this.
I believe this can be used as remote denial of service attack vector.
Possible solution for this would be limiting HTTP request body size (done by default in some servers like nginx), but this is unsuitable for cases where server needs to accept POST requests with big files (for example, file hosting service).
Similar case like this is net/http package's Server's MaxHeaderBytes field, which limits ammount of header data, but not body.

[andrius4669]

To illustrate issue, example client code exploiting this weakness: https://www.zerobin.net/?8db1a70f1608fb27#TzogmA1w8sPRNSnByLRcRrwHY0D7urscLFil6AbDdQA=
And test server's code affected by it: https://www.zerobin.net/?6f1057f7dc78ba55#4/v39opPJuwZyqEF8xft8Iqvmkv0QPyhWC189EZy+HE=
This can cause out of memory conditions therefore I recommend turning off swap on machines which run test server and running test server with nice -19.

[ianlancetaylor]

See also #20169.