Wire 1password SDK into Calamari

[rain-on]

This includes the following changes:

• introduces Octopus.1password.sdk as a dependency of Calamari.Testing
• Updates ExternalVariables.Get to fetch a secret from 1password if no env-var is avilable
• Adds "secret reference to the externalVariables documented in our ticket"

NOTE: the vault in 1password is not yet updated, thus all values will be taken from an env-var, not the vault.

[akirayamamoto]

My only concern is the Secret References are missing the field name, otherwise looks very similar to the OctopusDeploy repository from the top of my head.

[rain-on]

Overtaken by https://github.com/OctopusDeploy/Calamari/pull/1254, Will backport comments.

[rain-on]

overtaken by other PRs.

[APErebus]

@rain-on Can this be closed?