stratify 0.3.0

First public release. Continues the unpublished configkit under a name that is
available on crates.io and describes the crate rather than padding it.

Added

• Azure App Configuration source, behind the azure feature. The caller supplies
  the credential rather than the crate choosing one, so a service can use a managed
  identity in Azure and a developer credential locally. That keeps azure_identity
  out of this crate's dependency tree and lets the source be tested against a fake.
  Supports @nextLink pagination, an optional label filter, and the Database:Host
  key convention.
• ConfigBuilder::azure for the same, in the fluent style.
• #![forbid(unsafe_code)].

Changed

• Breaking: Source::load is now async. Implementations declare
  #[async_trait], and ConfigBuilder::build and ConfigStore::refresh are
  awaited. Network-backed sources should not block a runtime thread, and the
  synchronous trait left no way to use an async SDK without blocking inside a
  runtime, which panics.
• Replaced serde_yaml with serde_norway. serde_yaml is officially deprecated
  and should not be a dependency of a published crate.
• toml 0.8 to 1.
• Key nesting is shared between the environment and Azure sources rather than
  duplicated in each.

Migration from configkit 0.2

-let store = ConfigBuilder::default().json("base.json", 100).build()?;
+let store = ConfigBuilder::default().json("base.json", 100).build().await?;

For a custom source, add the attribute and the keyword:

+#[async_trait::async_trait]
 impl Source for MySource {
-    fn load(&self) -> Result<Value, ConfigError> { ... }
+    async fn load(&self) -> Result<Value, ConfigError> { ... }
 }