Add api to fetch short conn id len from a given channel/tserver

Add api to fetch short conn id len from a given channel/tserver

Need an api to fetch the configured conn id len for short headers, add
that in here

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/26592)