Resolve key vault secret in parallel

[zhiyuanliang-ms]

Why this PR?

#191

This change can bring about 40% speed-up when loading 10+ secrets.

Visible change

Added KeyVaultOptions.parallelSecretResolutionEnabled option to allow resolving key vault secret in parallel.

Usage:

const credential = new DefaultAzureCredential();
const appConfig = await load("endpoint", credential, {
  keyVaultOptions: { credential: credential,  parallelSecretResolutionEnabled: true}
});

About Key Vault throttling

Key Vault allows 4000 transactions in 10 seconds, per vault per region. ref

1. Key Vault SDK cannot be used in browser because of CORS. I have verified this. So, for the potential browser scenario of JS provider, people should not use Key Vault reference there. This significantly reduces the chance of Key Vault throttling as Key Vault reference is only valid to use in server scenario.

2. For potential throttling response from Key Vault, the best practice is to use the built-in retry policy configured through SecretClientOptions.

The provider supports either registering SecretClient instances or configuring SecretClientOptions. ref

About cache for Key Vault secret

Secret cache is added in #175