Add `SystemParam::to_read_only` to downgrade system params to shared form

## What problem does this solve or what need does it fill?

See [this Discord thread](https://discord.com/channels/691052431525675048/1066466054710251580/1066466054710251580) for a concrete example of a problem this would solve.

## What solution would you like?

Learn from `WorldQuery`, and add an associated type `ReadOnly` and a `to_read_only` method to the `SystemParam` trait.

## What alternative(s) have you considered?

This could be special-cased, but it seems that almost all system param are either read-only already, or have a read-only form.

Strange edge cases like `EventWriter` could just use `()` as their `ReadOnly` type.