feat: add ssrbaz block example + block autoloader

[chrisbellboy]

I recently needed to create some SSR blocks for a project and so just creating this PR in case it's also useful for the base repo.

Changes proposed in this Pull Request

• Add server-side rendered block example: ssrbaz.
• Add block autoloader to automatically register all blocks inside build/* - eliminates the need for manual register_block_type entries for each block. It also checks for the existence of SSR callbacks, specifically bpd_blocks_render_block_* and includes them via the $args if they are found.
• Add autoloader for PHP files inside individual block folders in src - allows for SSR blocks to have a PHP file with the render_callback alongside the other block files.