mixedversion: introduce deployment modes

[renatolabs]

This commit adds the notion of a "deployment mode" to mixedversion tests. Currently, we only define a system-only deployment mode, corresponding to the deployment performed on all mixedversion tests to date: cockroach is started without any tenants (virtual clusters), and client applications talk directly to the "system interface". This commit adds the functionality necessary for tests to be able to select what deployment modes make sense for them, if they verify something specific to some deployment mode.

In the future, more deployment modes will be added: tenants in shared-process mode, and external process (serverless-style) tenants.

Epic: none

Release note: None