Provide possibility to add default push rules for all users

This issue has been migrated from [#9325](https://github.com/matrix-org/synapse/issues/9325).

---

Based on the Matrix Server API spec the Synapse server provides some default push rules that apply to all users. There is currently no possibility to add a custom default push rule for all users. Therefore, it is necessary to define this rule for every user, which results in a lot of rule entries in the database, which could be avoided.