Adds a new sample fetch method to get the curve name used in the
key agreement to enable better observability. In OpenSSLv3, the function
`SSL_get_negotiated_group` returns the NID of the curve and from the NID,
we get the curve name by passing the NID to OBJ_nid2sn. This was not
available in v1.1.1. SSL_get_curve_name(), which returns the curve name
directly was merged into OpenSSL master branch last week but will be available
only in its next release.
