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.
