Add MultiProviderStrategy aliases

[EtaCassiopeia]

Summary

FeatureFlags.fromMultiProvider(providers, strategy) accepts a dev.openfeature.sdk.multiprovider.Strategy, so to pick a non-default strategy callers currently have to import the Java SDK package:

```scala
import dev.openfeature.sdk.multiprovider.FirstSuccessfulStrategy
fromMultiProvider(providers, new FirstSuccessfulStrategy)
```

This PR adds a small Scala-friendly facade so the same call becomes:

```scala
import zio.openfeature.MultiProviderStrategy
fromMultiProvider(providers, MultiProviderStrategy.firstSuccessful)
```

Changes

• New `core/src/main/scala/zio/openfeature/MultiProviderStrategy.scala` — exposes the two built-in strategies (`firstMatch`, `firstSuccessful`) plus a `Strategy` type alias.
• New `core/src/test/scala/zio/openfeature/MultiProviderStrategySpec.scala` — three tests pinning the aliases.

No edits to existing public API; pure addition.