Runtime config should support appending hooks

Currently, the runtime config file ~/.spacelift/config.yml overrides specified hooks

instead of appending, like contexts.

We use hooks baked into stacks for the following:

• azure SP provider credential env vars

• Vault provider credential env vars

• github token for read (for module source access)

These are implement via before_init hooks, this means that if any of our consumers specify before_init in runtime config, it breaks the integration with github , vault and azure.