hs: Client authorization with ADD_ONION seems to be ignored

From tor-dev: https://lists.torproject.org/pipermail/tor-dev/2021-May/014548.html

I'm one of the OnionShare developers and I'm trying to implement the new support for ClientAuthV3 via the controller as per [1] (thanks for adding it!). Since OnionShare depends on Stem, I also began by adding support for passing the ClientAuthV3 argument and V3Auth flag into Stem (I intend on submitting that as a PR once I solve the problem below, but I think the problem isn't Stem specific)

I can send the ClientAuthV3 base32-encoded public key and the V3Auth flag to ADD_ONION, and get a 250 response back.

The problem is that when I then visit the onion address, it doesn't actually require the Client Auth that was set :)

I am running the nightly Tor on Debian 10 (Buster):

Tor version 0.4.7.0-alpha-dev.
Tor is running on Linux with Libevent 2.1.8-stable, OpenSSL 1.1.1d, Zlib 1.2.11, Liblzma 5.2.4, Libzstd 1.3.8 and Glibc 2.28 as libc.
Tor compiled with GCC version 8.3.0

Steps to reproduce:

1. Take these public and private base32-encoded strings (as generated by [2], if you want to generate different ones)

public: FGTORMIDKR7T2PR632HSHLWA4G6HF5TCWSGMHDUU4LWBEFTAVYQQ

private: 5ZTNYVGHGMBCWT47YQT4ZFOFBWYU24C5PRQZ2CRCXZ5FKTVMJ7QA

2. Start a simple service on localhost:9735:

echo Hi | nc -l 127.0.0.1 9735

3. Connect to Tor's control port and add an onion with a private key that will derive the onion address rujvluxdgiibem3odopgkgiiajgtwfbdgkuqfyydhl5qupotpwyxjaid.onion (or put your own if you wish):

user@onionshare:~$ sudo telnet localhost 9051
Trying ::1...
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
authenticate ""
250 OK
ADD_ONION ED25519-V3:MNkxu0oI0CX6Oq1AEroRGSAiqXurEbzBdraDKJB1pkNkl9hNCr+bagdAg7gA4F3M/FrF7BHBdh5zdvkHB7oO4w== ClientAuthV3=FGTORMIDKR7T2PR632HSHLWA4G6HF5TCWSGMHDUU4LWBEFTAVYQQ Flags=V3Auth Port=80,9735
250-ServiceID=rujvluxdgiibem3odopgkgiiajgtwfbdgkuqfyydhl5qupotpwyxjaid
250-ClientAuthV3=AUEFTXH34ZVRXIIVOK5G7XLHTUXGVRLLXG7DG3NKJLRCVSEEHQDQ
250 OK

4. Visit http://rujvluxdgiibem3odopgkgiiajgtwfbdgkuqfyydhl5qupotpwyxjaid.onion and expect to get the Tor Browser pop-up dialog '[onion service] is requesting that you authenticate.. Enter your private key for this onion service'. etc

Instead: the service loads 'Hi' without any requirement for Client Auth occurring. I never added the private key to Tor Browser in any way.