Enhanced encoding option -- "separateanalyses" for `tm_t_binary_outcome`

[shajoezhu]

Background:

This issue can be traced back to https://github.com/insightsengineering/coredev-tasks/issues/287, were it was initially reported as "t_ef_rsp : Odds ratio and ORCI not matching to new STREAM 2.10 output, but all other results match"

After some investigation work, this is due to "It's just that STREAM fits different treatment groups into different logistic models, instead of fitting all levels into the same model", see details at https://github.com/insightsengineering/tern/issues/287#issuecomment-1331696426

Todo

Introduce an encoding option, to allow "separate analyses", for subgroup analysis see https://github.com/insightsengineering/tern/issues/287#issuecomment-1331696426