support http multipart request

[dakunge]

go-zero doesn't support uploading files well, need to modifies handler and logic files manually, eg: https://github.com/zeromicro/zero-examples/blob/main/http/upload/internal/logic/uploadlogic.go#L33

I found it's easy to support uploading files graceful and there will be no impact on existing functions and code

1. support file tag in api
2. a little change in genFile function
3. a little change in handler.tpl and logic.tpl

test api file:
`
syntax = "v1"

type (
UploadRequest {
File1 string file:"file1"
File2 string file:"file2"
Other string json:"string"
}
UploadResponse {}
)

service Test {
@handler Upload
post /upload (UploadRequest) returns (UploadResponse)
}

`

I think multipart request is standard http , we should support it.