Allow setting GIT_OPT_SET_SSL_CERT_LOCATIONS option in libgit2

[aharbis]

The git2::opts module exposes some options, but not all. I'm interested in being able to set GIT_OPT_SET_SSL_CERT_LOCATIONS to control where libgit2 looks for SSL certificates.

If there is some workaround (maybe a build-time configuration?) that would also be helpful.

[ehuss]

A PR to add a binding for that option would be welcome. I think it would just roughly follow the form of the other options.

[aharbis]

PR raised: https://github.com/rust-lang/git2-rs/pull/997