Support multiple default session images

[ian-flores]

See discussion: https://positpbc.slack.com/archives/C05HKJEGELB/p1734042466074649

Let's add the launcher-sessions-container-images field to our Workbench config so we can have a list of default images rather than just one.
Docs: https://docs.posit.co/ide/server-pro/reference/rserver_conf.html#launcher-sessions-container-images

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Migrated from rstudio/ptd#1585