Add append_header to RequestBuilder or make header types fully public

Currently, if i want to use RequestBuilder with multiple values for the same header, i have to construct a weird iterator using HeaderValue::from_str since the necessary tools to do this are missing.

I cannot:
- simply loop and call append_header, since RequestBuilder doesn't have this
- construct a Headers, since its private and i can't pass it to RequestBuilder
- construct and HeaderValues for RequestBuilder::header since its private

The only way i want to have multiple header values with RequestBuilder, i need to do this:
```rs
builder = builder.header(
name,
vec!["a", "b"]
.iter()
.map(|f| HeaderValue::from_str(f).unwrap().collect()),
);
```

which is quite clunky

Start by locating RequestBuilder, Headers, and HeaderValue in the Surf source and inspect how the existing header method accepts values. Compare adding append_header with exposing the private types, then identify the API behavior needed for multiple values under one header name; done means callers no longer need the workaround shown in the issue.

Written by the indexing model from the issue text.