Bug 5558: Add AKI to client-first SslBump certificates

[ccijunk]

Summary

Certificates generated by SslBump in client-first mode carried
no Authority Key Identifier (AKI) extension:

• mimicAuthorityKeyId() returned early whenever
  mimicCert was absent, and
• its only caller sat inside the
  if (properties.mimicCert.get()) branch of
  buildCertificate().

In client-first bump Squid completes the client handshake
before contacting the origin, so properties.mimicCert is
null and the generated leaf had no AKI at all. Strict X.509
validators therefore reject every forged leaf on every bumped
connection, even though the chain is otherwise fully trusted:

• Python 3.13+ with VERIFY_X509_STRICT (the default for
  conda 26+) fails the TLS handshake with Missing Authority Key Identifier;
• openssl verify -x509_strict reports error 85 at 0 depth lookup: Missing Authority Key Identifier.

The failure is mode-dependent and therefore unintended: the
same proxy, signing CA and client produce a valid certificate
in server-first mode and an invalid one in client-first mode.

The origin certificate is not needed to build the extension.
The value of the keyIdentifier field is taken from
issuerCert; mimicCert only decided the shape (whether
to write keyIdentifier, and whether to also write
authorityCertIssuer plus authorityCertSerialNumber).
Per RFC 5280 section 4.2.1.1, the keyIdentifier field is
required in certificates generated by a conforming CA
(except for self-signed certificates), and that exception
does not apply because the signing CA has a Subject Key
Identifier.

Changes

• Rename mimicAuthorityKeyId() to addAuthorityKeyId():
  Squid mimics the extension presence, but the extension
  value is always built from the signing CA rather than
  copied, so the old name was misleading.
• Move the !addKeyId && !addIssuer check to the top level
  of the function so it covers all current and future paths.
• Call mimicExtensions() unconditionally from
  buildCertificate() and make mimicExtensions() tolerate
  a nil mimicCert, instead of adding a second, conditional
  call site.
• Fix the mimicExtensions() description comment (it
  said extensions were copied "from cert to
  mimicCert" - backwards).

The mimicking path behavior is unchanged: with mimicCert
present the same fields are read from the origin
certificate, the same early return applies, and the
value-construction section is untouched.

Testing

With a client-first bump configuration (ssl_bump bump step1):

• openssl x509 -text on the captured leaf shows
  Authority Key Identifier: keyid:<signing CA's SKI>,
  matching the Subject Key Identifier of the CA certificate
• openssl verify -x509_strict -CAfile squid-ca.pem leaf.pem returns OK
• A Python 3.14 client using VERIFY_X509_STRICT
  completes the TLS handshake through the proxy
• server-first mode output is unchanged (mimic path
  untouched)
• Full source build (Ubuntu 22.04, gcc 11, OpenSSL 3.0)

Note: the AKI is the only extension that needs to be added -
the generated leaf still lacks key usage, extended key usage,
basic constraints and a Subject Key Identifier, and strict
validation does not object to any of those.

[rousskov]

@ccijunk, thank you for sharing this fix! Please add yourself to the CONTRIBUTORS file as a part of this PR (or let us know if you prefer not to do that -- a manual action will be needed to bypass the corresponding CI check in that case).

P.S. No need to squash or rebase your PR branch as you add commits. PR commits will be squashed automatically when merging your PR.