Use environment variables when creating signatures

When creating an action signature (e.g. for a commit author and
committer) read the following environment variables that can override
the configuration options:

 * `GIT_AUTHOR_NAME` is the human-readable name in the "author" field.
 * `GIT_AUTHOR_EMAIL` is the email for the "author" field.
 * `GIT_AUTHOR_DATE` is the timestamp used for the "author" field.
 * `GIT_COMMITTER_NAME` sets the human name for the "committer" field.
 * `GIT_COMMITTER_EMAIL` is the email address for the "committer" field.
 * `GIT_COMMITTER_DATE` is used for the timestamp in the "committer"
   field.
 * `EMAIL` is the fallback email address in case the user.email
   configuration value isn't set. If this isn't set, Git falls back to
   the system user and host names.

This is taken from the git documentation chapter "10.8 Environment
Variables":

https://git-scm.com/book/en/v2/Git-Internals-Environment-Variables

This PR adds support for reading these environment variables by adding
two new functions `git_signature_default_author` and
`git_signature_default_committer` and deprecates the
`git_signature_default` function.

Fixes: https://github.com/libgit2/libgit2/issues/3751

Prior work:
 * https://github.com/libgit2/libgit2/pull/4409
 * https://github.com/libgit2/libgit2/pull/5479
 * https://github.com/libgit2/libgit2/pull/6290

examples/tag.c

@@ -226,7 +226,7 @@ static void action_create_tag(tag_state *state)
 	check_lg2(git_revparse_single(&target, repo, opts->target),
 			"Unable to resolve spec", opts->target);
 
-	check_lg2(git_signature_default(&tagger, repo),
+	check_lg2(git_signature_default_author(&tagger, repo),
 			"Unable to create signature", NULL);
 
 	check_lg2(git_tag_create(&oid, repo, opts->tag_name,