remote: introduce remote_ready_cb, deprecate resolve_url callback

opts: test GIT_OPT_SET_SSL_CERT_LOCATIONS

fixes #6003

Include a self-signed certificate for test.libgit2.org:1443 that we can
use to verify that GIT_OPT_SET_SSL_CERT_LOCATIONS works.

Checkout dry-run

midx: Introduce git_odb_write_multi_pack_index()

Use error code GIT_EAUTH for authentication failures

midx: Add a way to write multi-pack-index files

Using a callback to set a resolve_url is not particularly idiomatic.
Deprecate it in favor of the `set_instance_url` and
`set_instance_pushurl` functions which can now be called from the
`git_remote_ready_cb` callback.

Introduce a new callback that fires when the remote is ready to connect.

WIP: .devcontainer: settings for a codespace workflow

CMake. hash sha1 header has been added.

Co-authored-by: Edward Thomson <ethomson@github.com>

Users may want to override the URL on a particular instance of a remote,
instead of updating the configuration.  Previously, users could use a
callback to do this, but this is not particularly idiomatic.

git_array_alloc: return objects of correct type

Homogenize semantics for atomic-related functions

This change introduces git_odb_write_multi_pack_index(), which creates a
`multi-pack-index` file from all the `.pack` files that have been loaded
in the ODB.

Fixes: #5399

This change adds the git_midx_writer_* functions to allow to
write and create `multi-pack-index` files from `.idx`/`.pack` files.

Part of: #5399

Set refs/remotes/origin/HEAD to default branch when branch is specified

There were some subtle semantic differences between the various
implementations of atomic functions. Now they behave the same, have
tests and are better documented to avoid this from happening again in
the future.

Of note:

* The semantics chosen for `git_atomic_compare_and_swap` match
  `InterlockedCompareExchangePointer`/`__sync_cal_compare_and_swap` now.
* The semantics chosen for `git_atomic_add` match
  `InterlockedAdd`/`__atomic_add_fetch`.
* `git_atomic_swap` and `git_atomic_load` still have a bit of semantic
  difference with the gcc builtins / msvc interlocked operations, since
  they require an l-value (not a pointer). If desired, this can be
  homogenized.

This reverts commit 487f2a82, reversing
changes made to c6cf7f0e.

array: check dereference from void * type

GCC C11 warnings

Instead of buf->"typeofbuffer"ReparseBuffer the members will be
referenced with buf->ReparseBuffer."typeofbuffer"

https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/ns-ntifs-_reparse_data_buffer?redirectedfrom=MSDN
calls the union DUMMYUNIONNAME but that looks a bit cluttered.

Introduce GIT_WARN_UNUSED_RESULT

open: input validation for empty segments in path