[5.x] Planned work for the next major

[cleptric]

This is a rough outline about the changes we're planning to make in the next major version of the PHP SDK.

• Remove remaining Metrics code
• Remove the Hub and implement the new Global, Isolation and Current Scope concept
• Remove Transactions and implement a Spans Only API
• New propagators
• Explore supporting Attachments
• Better Pii controls, instead of send_default_pii
• Remove symfony/options-resolver
• Remove nullable clients on the hub
• Make sure the SDK is fully compatible with async PHP runtimes like FrankenPHP
• Ship an early access version of the sentry-php-tracer extension

[cleptric]

We might also consider bumping the PHP version, based on current stats for 4.x.

https://packagist.org/packages/sentry/sentry/php-stats#4