Fixes from code analysis

buf: deprecate public git_buf writing functions

filter: filter drivers stop taking git_buf as user input

Introduce `create_commit_cb`, deprecate `signing_cb`

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

A `git_buf` is now a read-only structure as far as callers are
concerned.  This is a mechanism that we can return data to callers
using memory that is owned by the library and can be cleaned up by
callers (using `git_buf_dispose`).

A `git_buf` can no longer be allocated by callers or provided to the
library.

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.

The signing callback should not be used; instead, callers should provide
a commit_create_cb, perform the signing and commit creation themselves.

Introduce a new mechanism for `git_rebase_commit` for callers to
customize the experience.  Instead of assuming that we produce the
commit for them, provide a commit creation callback that allows callers
to produce the commit themselves and return the resulting commit id.

WIP: .devcontainer: settings for a codespace workflow

CMake. hash sha1 header has been added.

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