Provide an interface around OpenSSL to dynamically load the libraries and symbols,
so that users can distribute a libgit2 library that is not linked directly against
OpenSSL.  This enables users to target multiple distributions with a single binary.

This mechanism is optional and disabled by default.  Configure cmake with
-DUSE_HTTPS=OpenSSL-Dynamic to use it.

Refactor the OpenSSL stream implementation so that the legacy code is better
abstracted.  This will enable future development.

http: don't require a password

Respect the force flag on refspecs in git_remote_fetch

Attempt authentication when a username is presented but a password is
not; this can happen in particular when users are doing token
authentication and specifying the token in the URL itself.  For example,
`https://token@host/` is a valid URI and should be treated as a username
of `token` with an empty password.

Introduce GIT_OPT_SET_ODB_LOOSE_PRIORITY and GIT_OPT_SET_ODB_PACKED_PRIORITY
to allow overriding the default priority values for the default ODB
backends. Libgit2 has historically assumed that most objects for long-
running operations will be packed, therefore GIT_LOOSE_PRIORITY is
set to 1 by default, and GIT_PACKED_PRIORITY to 2.
When a client allows libgit2 to set the default backends, they can
specify an override for the two priority values in order to change
the order in which each ODB backend is accessed.