Extend rego package to let caller set random seed

[tsandall]

The topdown package exposes a random seed parameter however it's not plumbed into the rego package which means that callers can't control it.

We should extend the rego package to let users set the seed:

https://pkg.go.dev/github.com/open-policy-agent/opa/topdown#Query.WithSeed

The changes required in the rego package a minimal:

• Add new parameters (one for rego.New() and another for prepared queries) to provide the seed reader
• Pass the seed reader into topdown

The changes in rego.go from this which added support for setting the wallclock time are a good model to follow.