Allow set timeout for SSH connection.

[SuperBo]

Following documentation, there is no way to set timeout for ssh connection.

GIT_OPT_SET_SERVER_CONNECT_TIMEOUT: Sets the timeout (in milliseconds) to attempt connections to > a remote server. This is supported only for HTTP(S) connections > and is not supported by SSH. Set to 0 to use the system default. > Note that this may not be able to be configured longer than the > system default, typically 75 seconds.

Reproduction steps

Expected behavior

This timeout should also apply SSH connection. Libssh2 has an API for setting ssh connection timeout.
https://libssh2.org/libssh2_session_set_timeout.html

Actual behavior

git_remote_fetch hang forever if can't connect to the repository.

Version of libgit2 (release number or SHA1)

v1.7.1

Operating system(s) tested

Mac OS