Add lsquic_conn_get_full_cert_chain to obtain certs including leaf on server side

[richard-ramos]

Adds an additional function to the API: lsquic_conn_get_full_cert_chain, which can be used when you want the client to send a certificate, and then perform a custom validation over the certificate sent by the client.

The reason behind why this function is necessary is because lsquic_conn_get_server_cert_chain will return the cert chain excluding the leaf when called from the server, and if the client is using a self signed certificate, the cert will just no appear there.

[dtikhonov]

Thank you for PR! Any chance you could add this call to bin/http_server.c? (It's fine if the answer is "no," but that will make this PR slower to merge, because then I will add this code to test...)

[richard-ramos]

Sure! I'll update the PR today/tomorrow. Thank you!