Add support for expect and ssh2 using shivammathur/extensions on macOS

Add config to brew extensions and refactor add_brew_extension in darwin.sh to use it

src/configs/brew_extensions

@@ -0,0 +1,28 @@
+amqp=amqp
+apcu=apcu
+expect=expect
+grpc=grpc
+igbinary=igbinary
+imagick=imagick
+imap=imap
+memcache=memcache
+memcached=memcached
+mongodb=mongodb
+msgpack=msgpack
+pcov=pcov
+pecl_http=http
+phalcon3=phalcon
+phalcon4=phalcon
+propro=propro
+protobuf=protobuf
+psr=psr
+raphf=raphf
+rdkafka=rdkafka
+redis=redis
+ssh2=ssh2
+swoole=swoole
+vips=vips
+xdebug=xdebug
+xdebug2=xdebug
+yaml=yaml
+zmq=zmq