add support for using mbedtls instead of openssl

[keredson]

this library was recently updated to use libssh v0.10.6. libssh v0.10.0 added support for using mbedtls (instead of openssl). can we add a feature to enable building libssh with option WITH_MBEDTLS? (vendored-mbedtls perhaps?)

See:

• https://www.libssh.org/2022/08/26/libssh-0-10-0/
• https://github.com/xbmc/libssh/blob/master/README.mbedtls
• https://github.com/xbmc/libssh/blob/3cb0a1bf023cce7209290f998b8dc3029ed3df77/DefineOptions.cmake#L10C1-L11C1