Add experimental package2.xml for install-php-extensions

[vlastv]

Adds a draft package2.xml to test installation via install-php-extensions script.

Tested and works in php:8.4-alpine Docker.

File is a quick prototype: tags contain placeholder data and require full update.

Build from php:8.4-alpine with install-php-extensions
Run install-php-extensions vlastv/php-blake3@pear

Verify extension loads php -m | fgrep -i blake3

# php -r "echo blake3('Hello world', 20),\"\n\";"
e7e6fb7d2869d109b62cdb1227208d4016cdaa0a