Accept an enum (`git_stream_t`) during custom stream registration that
indicates whether the registration structure should be used for standard
(non-TLS) streams or TLS streams.

Update the new stream registration API to be `git_stream_register`
which takes a registration structure and a TLS boolean.  This allows
callers to register non-TLS streams as well as TLS streams.

Provide `git_stream_register_tls` that takes just the init callback for
backward compatibliity.

For testing, we may wish to use a man-in-the-middle proxy that can
inspect the CONNECT traffic to our test endpoints.  For this, we will
need to accept the proxy's certificate, which will not be valid for the
true endpoint.

Add a new environment variable, GITTEST_REMOTE_SSL_NOVERIFY to disable
https certificate validation for the tests.

Introduce `git_tls_stream_wrap` which will take an existing `stream`
with an already connected socket and begin speaking TLS on top of it.
This is useful if you've built a connection to a proxy server and you
wish to begin CONNECT over it to tunnel a TLS connection.

Also update the pluggable TLS stream layer so that it can accept a
registration structure that provides an `init` and `wrap` function,
instead of a single initialization function.

Rename credential callback to proxy_cred_cb to match new cert callback.

Give the proxy tests a proxy certificate callback, and allow self-signed
certificates when the `GITTEST_REMOTE_PROXY_SELFSIGNED` environment
variable is set (to anything).  In that case, simply compare the hostname
from the callback to the hostname that we connected to.

As we want to support HTTPS proxies, support an optional
`GITTEST_REMOTE_PROXY_SCHEME` environment variable for tests that will
allow for HTTPS support.  (When unset, the tests default to HTTP
proxies.)

Change the `GITTEST_REMOTE_PROXY_URL` environment variable to be
`GITTEST_REMOTE_PROXY_HOST`, since it is a host:port combination, not an
actual URL.  (We cannot use a URL here since we may want to include the
username:password combination in the constructed URL.)

index: introduce git_index_iterator