feat: Add the ability to get the peer certificate of an SSL connection

[dirkx]

Summary

New feature: Add the ability to get the peer certificate of an SSL connectio. This is useful for IoT when the root/cert trust chain has a shorter lifecylce than the device itself. Includes example code.

It adds two methods to the WiFiClientSecure client:

1. bool getFingerprintSHA256( uint8_t fingerprint_remote_sha256[32]) -- return true and the fingerprint (i.e. the SHA256 of the raw x509 as a DER - identical to what you see in for example your webbrowser). Or false on error.
2. const mbedtls_x509_crt* getPeerCertificate(); -- return the actual X509 struct or NULL on error.

Impact

No impact; backwards compatible (only adds to the API)

[CLAassistant]

All committers have signed the CLA.

[me-no-dev]

Thanks @dirkx :) nice feature!

[dirkx]

Thank you for the quick merge !