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>

The filter's proxy_stream is used to adapt filters that only provided an
`apply` function into a `stream` function.  Make this internal to the
library instead of private to the filter file.  This will allow the
filters to use it directly, instead of relying on the filter
functionality to do the proxying.