Previously, parsing failures and the header not existing made
extract_header_list_values return an empty Optional, making it
impossible to differentiate between the two.
Required for implementing CORS-preflight, where parsing failures for
the headers makes it fail, but not having them doesn't make it fail in
all cases.
We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
The header-specific ABNF rules are completely ignored for now, but we
can at least extract a single header value, which at least works for
simple cases like `Location`-based redirects.
This is the way.
On a more serious note, there's no reason to keep adding ref-counted
classes to LibWeb now that the majority of classes is GC'd - it only
adds the risk of discovering some cycle down the line, and forces us to
use handles as we can't visit().
This allows us to use this:
```cpp
auto header = TRY_OR_RETURN_OOM(realm,
Infrastructure::Header::from_string_pair(name, value));
```
Instead of the somewhat unwieldly:
```cpp
auto header = Infrastructure::Header {
.name = TRY_OR_RETURN_OOM(realm, ByteBuffer::copy(name.bytes())),
.value = TRY_OR_RETURN_OOM(realm, ByteBuffer::copy(value.bytes())),
};
```
This is needed to eventually share a header list between a Request or
Response object's internal infra request/response and the object's
exposed Header object.
The Fetch spec unfortunately will cause a name clash between the Request
concept and the Request JS object - both cannot live in the Web::Fetch
namespace, and WrapperGenerator generally assumes `Web::<Name>` for
things living in the `<Name>/` subdirectory, so let's instead move infra
code into its own namespace - it already sits in a (sub-)subdirectory
anyway.
