Adding a new session value 'tenancy_impersonated_by' when you are impersonating a tenant.

[nathandaly]

This PR adds a new session value tenancy_impersonated_by in the tenant scope when you are impersonating a tenant.

This is similar to how Laravel Nova adds the nova_impersonated_by session value and could be used to identify when you are impersonating, who is impersonating. This may be useful for additional application logic and UI niceties such as an impersonation banner.

[nathandaly]

Docker container failed to initialise.

[stancl]

Just for some context — we're looking into this internally, since we were just about to work on something very similar. Using a session in the central context (or on the central domain) is potentially problematic, so we're going through the different solutions that could be used here to see what will work.

[stancl]

Resolved in #1029