Use explicit fetching for digests in cryptohash (OpenSSL >= 3.0)

cryptohash_openssl.c initialized the EVP_MD_CTX with the implicit static
digest objects (EVP_sha256() and friends).  These do not dispatch
through a loaded provider.  OpenSSL 3.0 and newer versions recommend to
switch to an explicit fetch, using EVP_MD_fetch(), the older routines
being deprecated, available for backward-compatibility purposes.

Now, when building with OpenSSL 3.0 or a newer bersion, we fetch the
digest by name with EVP_MD_fetch(), cache it in the context, and free it
on teardown.  This has as consequence to feed the hash from a provider.
The implicit path is kept for older OpenSSL, 1.1.1 being the oldest
version still supported on HEAD, and for LibreSSL.

This is a slight change in the hashing logic.  No backpatch is done as
that looks safer, and the code in the back-branches is also able to
work.  This has been locally tested across OpenSSL 1.1.1 up to 3.5, so
hopefully nothing is broken.

Note: The submitted code was clearly AI-generated.  I have spend a bit
of time cleaning it up, making it more consistent with the project style
in terms of coding, adjusting a few more things on the way.

Author: Mark Atwood <[email protected]>
Co-authored-by: Michael Paquier <[email protected]>
Discussion: 
https://postgr.es/m/[email protected]

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b91f79cd08abba7a800d3faf79bb96e8adc8752e

Modified Files
--------------
src/common/cryptohash_openssl.c | 47 +++++++++++++++++++++++++++++++++++++++++
1 file changed, 47 insertions(+)

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