Use EVP_MAC for HMAC with OpenSSL 3.0 and later

The attached patch switches src/common/hmac_openssl.c to the EVP_MAC API when
building against OpenSSL 3.0 or newer.

The legacy HMAC_CTX interface (HMAC_CTX_new, HMAC_Init_ex, HMAC_Update,
HMAC_Final) has been deprecated since 3.0, and it does not dispatch through the
provider framework, so a loaded provider's HMAC implementation is bypassed.
The patch fetches "HMAC" with EVP_MAC_fetch(), creates an EVP_MAC_CTX, and
selects the digest through an OSSL_PARAM. The HMAC_CTX path is retained for
older OpenSSL and for LibreSSL, guarded by OPENSSL_VERSION_NUMBER >=
0x30000000L.

The fetch uses the default library context and a NULL property query, so no
dependency is added and no particular provider is required; the provider is
selected by the usual OpenSSL configuration (openssl.cnf).

When the pg_hmac abstraction was built, EVP_MAC was set aside deliberately --
"a bit too new to use though, as we need to support OpenSSL down to 1.0.1 on
HEAD ... So instead I have decided to rely on the older interface based on
HMAC_Init_ex()" [1]. That constraint no longer holds: the minimum supported
OpenSSL is now 1.1.1, and the version guard confines this change to 3.0+.
Dropping use of an interface deprecated in 3.0 also stands on its own,
independent of provider routing.

SCRAM authentication does no crypto of its own -- it rides pg_hmac -- so it
follows the active provider once the backend does.

I checked what this path resolves to at run time using
EVP_MAC_get0_provider(). On OpenSSL 3.0.13:

legacy HMAC_CTX / HMAC_Init_ex : legacy low-level API, not
provider-dispatched
EVP_MAC_fetch(NULL,"HMAC",NULL) : default

Details:

* Against master, tested at 8b73ceb78f. It touches only
src/common/hmac_openssl.c and applies on its own; there is no dependency on
the two related patches I am posting in separate threads.

* Built and tested with OpenSSL 3.0.13 on Ubuntu 24.04 (x86-64): clean build,
src/test/regress, src/test/ssl and src/test/authentication all pass. HMAC
output was also checked against the RFC 4231 test vectors.

* No new regression tests. This replaces the implementation behind pg_hmac
without changing its behavior or API, and the existing SCRAM coverage in
src/test/authentication exercises it.

* No documentation change.

* No performance impact expected. EVP_MAC_fetch() does a provider lookup,
but the fetched object is cached in the context, and this path runs at
connection time rather than in any tight loop.

This was previously posted as a three-patch series in a single thread [2].
Reposting as separate threads with the patch attached, per review request.

Intended for the next commitfest.

[1] https://postgr.es/m/X9m0nkEJEzIPXjeZ@paquier.xyz
[2] https://postgr.es/m/20260805004805.1174492-1-mark%40reviewcommit.com

--
Mark