Dispose KeyVaultClient after use

We create a `KeyVaultClient` here:

https://github.com/corvus-dotnet/Corvus.Extensions.CosmosDb/blob/04b750837fd6550068315da6f21f2c798bf03948/Solutions/Corvus.SpecFlow.Extensions.CosmosClient/Corvus/SpecFlow/Extensions/SecretHelper.cs#L103

But we do not dispose it. We should work out whether disposing in situ is fine, or whether this is one of those more problematic scenarios where this is essentially an `HttpClient` in disguise that we need to hang onto and reuse.