ssh: include sha256 host key hash when supported

Various examples shape-ups

Improve trace support in tests

Move `git_off_t` to `git_object_size_t`

Add compat typdef for git_attr_t

Some libraries haven't updated to git_attr_value_t and break.  Adding
the comapt typedef as suggested.

Only show test trace execution when the CLAR_TRACE_TESTS environment
variable is set.  This reduces the noise during tracing.

Tracing is meant to be extremely low-impact when not enabled.  We
currently ship no tracing calls in libgit2, but if / when we do, the
tracing infrastructure is created to skip tracing as quickly as
possible.  It should compile to a simple test when tracing is off.

Thus, there's on reason to not enable it by default.

CI Build Updates

Prefer `off64_t` internally.

Use int64_t internally for type visibility.

Prefer `off64_t` to `git_off_t` internally for visibility.

Prefer `off64_t` to `git_off_t` for internal visibility.

64 bit types are always 64 bit.

Provide usage hints to valgrind.  We trust the data coming back from
OpenSSL to have been properly initialized.  (And if it has not, it's an
OpenSSL bug, not a libgit2 bug.)

We previously took the `VALGRIND` option to CMake as a hint to disable
mmap.  Remove that; it's broken.  Now use it to pass on the `VALGRIND`
definition so that sources can provide valgrind hints.

valgrind will warn that OpenSSL will use undefined data in connect/read
when talking to certain other TLS stacks.  Thankfully, this only seems
to occur when gcc is the compiler, so hopefully valgrind is just
misunderstanding an optimization.  Regardless, suppress this warning.

We currently talk to Azure Repos for executing an online test
(online::clone::path_whitespace).  Add a simpler test to talk to Azure
Repos to make it obvious that strange test failures are not likely the
whitespace in the path, but actually a function of talking to Azure
Repos itself.

Our docker builds are getting expensive, let's cache some of this.

The valgrind in the PPA is broken and ignores `--exit-errorcode`.
Build and install our own.

Use a multi-stage docker build so that we can cache early stages and not
need to download the apt-provided dependencies during every build (when
only later stages change).

Deleting the apt cache can be helpful for reducing the size of a
container, but since we don't push it anywhere, it only hinders our
ability to debug problems while working on the container.  Keep it.

libcrypto will read uninitialized memory as entropy.  Suppress warnings
from this behavior.

The lsb-release command is missing on our images; just show the
information from the file instead of relying on it.

Provide a sane default for `CMAKE_GENERATOR` in the build script so that
it can be invoked without having to set that in the environment.

Instead of using a signed type (`off_t`) use an unsigned `uint64_t` for
the size of the files.

Instead of using a signed type (`off_t`) use a new `git_object_size_t`
for the sizes of objects.

Instead of using a signed type (`off_t`) use `uint64_t` for the maximum
size of files.

Instead of using a signed type (`off_t`) use a new `git_object_size_t`
for the sizes of objects.

Instead of using a signed type (`off_t`) use a new `git_object_size_t`
for the sizes of objects.

Introduce `git_object_size_t`, an unsigned type that we can use for the
maximum size of git objects.