Add switch to run `update-alternatives` to set the managed PHP version as default

[mpdude]

Depending on which PHP version is installed with this module (and there might even be other versions not managed by it!), it's hard to predict what /usr/bin/php will resolve to.

At least on Ubuntu, the update-alternatives command can be used to control that. You need to be aware, however, that there are a few alternatives that need to be configured:

$ update-alternatives  --get-selections | grep ph
phar                           auto     /usr/bin/phar8.1
phar.phar                      auto     /usr/bin/phar.phar8.1
php                            auto     /usr/bin/php8.1
php-config                     auto     /usr/bin/php-config8.1
phpize                         auto     /usr/bin/phpize8.1

It would be nice if this module could encapsulate the necessary knowledge (which alternatives need to be set to what). Maybe an additional module (puppet/alternatives?) comes in handy?

class { '::php': update_alternatives => true } should set all php-related alternatives to the PHP version given in ::php::globals.

X-Ref #235