bpftool: add bash completions for btf command

Add full support for btf command in bash-completion script.

Cc: Quentin Monnet <quentin.monnet@netronome.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>