[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 the Hub and implement the new Global, Isolation and Current Scope concept
• [ ] Remove Transactions and implement a Spans Only API
• [ ] Remove remaining Metrics code
• [ ] New sampling controls
• [ ] Explore supporting Attachments
• [ ] Better Pii controls, instead of send_default_pii
• [ ] Remove symfony/options-resolver
• [ ] Ship an early access version of the sentry-php-tracer extension
• [ ] Remove nullable clients on the hub

[cleptric]

~~We might also consider bumping the PHP version, based on current stats for 4.x.~~ We continue to support PHP 7.2.

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