Add peer_cert_serial_number and fix validity

[razvanphp]

Proposed Changes

Fix the peer cert end date rendered as a tuple
X.509 encodes dates two different ways depending on whether the year fits in two digits:

• UTCTime — YYMMDDHHMMSSZ, used for years 1950–2049.
• GeneralizedTime — YYYYMMDDHHMMSSZ, used for 2050 and later.

Types of Changes

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• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

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Further Comments

We run a fairly large mTLS fleet of IoT devices and the serial number is a must to be able to check the exact cert a device is using or if it rotated it's cert correctly. I'm sure it will come in handy for other users as well.

[michaelklishin]

Moved to #16460.