MDEV-31669: use sha2-256 as a digest instead of md5

[MohamedM216]

Jira Issue number for this PR: MDEV-31669

I followed mysql implementation as suggested in jira.

PERFORMANCE SCHEMA MD5 DIGEST NEEDS TO CHANGE DIGEST FOR FIPS
COMPLIANCE
Before this fix, DIGEST hashes are computed using the MD5 hash.
MD5 is not FIPS compliant.
This fix replaces the MD5 hash with SHA256 (which is compliant).
As a result, DIGEST columns are changed from VARCHAR(32) to VARCHAR(64)

[gkodinov]

This is a preliminary review. Right now there's no strong opposition against removing MD5 from this. But the choice of a replacement hash function is yet to be determined. I can see the logic in Sergei's argument that using a crypto hash for a non-crypto purpose is kind of a mismatch. But I will leave that discussion for the final review now.

First of all: please make sure the change compiles and runs all buildbot test in a satisfactory way.

[gkodinov]

sha2 is causing a compile error. Please fix that.

[MohamedM216]

The sha2.h header exists locally on my machine, so following the mysql implementation worked fine for my local build. However, I switched to evp.h to ensure a consistent and portable header across different build environments.