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

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

Start at the ::php class parameter and ::php::globals version selection described in the issue, then trace how platform-specific PHP packages are managed. Done means an opt-in update_alternatives setting configures the listed PHP-related alternatives to the selected PHP version.