Adds a new experimental `pulumi watch` CLI command which can be used for inner loop development on a Pulumi stack. This command is only available currently via `PULUMI_EXPERIMENTAL=true` while in active development.
The `watch` command does the following:
1. Watches the workspace (the tree rooted at the `Pulumi.yaml` file) for changes
2. Triggers an `update` to the stack whenever there is a change
3. Streams output containing summaries of key update events as well as logs from any resources under management into a combined CLI output
Part of https://github.com/pulumi/pulumi/issues/3448.
The PULUMI_EXPERIMENTAL flag also makes`query` and `policy` available.
The current pattern of using backend.Stack values in the CLI but
accepting backend.StackReference values in backend methods leads to
frequent transitions between these types. In the case of the HTTP
backend, the transition from a StackReference to a Stack requires an API
call. These changes refactor the backend.Backend API such that most of
its methods accept backend.Stack values in place of
backend.StackReference values, which avoids these transitions.
This removes two calls to the getStack API on the startup path of a
`pulumi preview`.
