MEDIUM: h3: use absolute URI form with :authority

MEDIUM: h3: use absolute URI form with :authority

Change the representation of the start-line URI when parsing a HTTP/3
request into HTX. Adopt the same conversion as HTTP/2. If :authority
header is used (default case), the URI is encoded using absolute-form,
with scheme, host and path concatenated. If only a plain host header is
used instead, fallback to the origin form.

This commit may cause some configuration to be broken if parsing is
performed on the URI. Indeed, now most of the HTTP/3 requests will be
represented with an absolute-form URI at the stream layer.

Note that prior to this commit a check was performed on the path used as
URI to ensure that it did not contain any invalid characters. Now, this
is directly performed on the URI itself, which may include the path.

This must not be backported.