Add support for multiple key shares

Add support for multiple key shares

This PR is the implementation of concluded discussion that occurred in a
draft PR #25605. This changes were mainly authored by @martinschmatz
with some contribution from myself.

It addresses issue #21633

This extends the group list definition to support a more complex
definition while still retaining backward compatibility with the simple
form of colon separated groups.

Details of the agreed format and expected behaviour can be found in
#25605 and in the documentation changes.

Signed-off-by: Dave Kelsey <d_kelsey@uk.ibm.com>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/26445)