This change allows using the passphrase secrets manager when creating
a stack managed by the Pulumi service. `pulumi stack init`, `pulumi
new` and `pulumi up` all learned a new optional argument
`--secrets-provider` which can be set to "passphrase" to force the
passphrase based secrets provider to be used. When unset the default
secrets provider is used based on the backend (for local stacks this
is passphrase, for remote stacks, it is the key managed by the pulumi
service).
As part of this change, we also initialize the secrets manager when a
stack is created, instead of waiting for the first time a secret
config value is stored. We do this so that if an update is run using
`pulumi.secret` before any secret configuration values are used, we
already have the correct encryption method selected for a stack.
We move the implementations of our secrets managers in to
`pkg/secrets` (which is where the base64 one lives) and wire their use
up during deserialization.
It's a little unfortunate that for the passphrase based secrets
manager, we have to require `PULUMI_CONFIG_PASSPHRASE` when
constructing it from state, but we can make more progress with the
changes as they are now, and I think we can come up with some ways to
mitigate this problem a bit (at least make it only a problem for cases
where you are trying to take a stack reference to another stack that
is managed with local encryption).
