adding provide to php composer.json

Adding composer config to allow the native protobuf extension to provide ext-protobuf.
This allows libraries to require at least one protobuf implementation. If the extension
is not available, it can be provided by the native package. If the extension is available
but the native package is required, the native will be installed.
Importantly, for libraries which require at least one of them to be installed, composer
will complain if neither is available.

php/composer.json

@@ -8,6 +8,9 @@
   "require": {
     "php": ">=7.0.0"
   },
+  "provide": {
+    "ext-protobuf": "*"
+  },
   "require-dev": {
     "phpunit/phpunit": ">=5.0.0 <8.5.27"
   },