[Infra UI] Architecture discussion: Implicit or explicit source configuration selection

[weltenwort]

Currently, the Logs UI (and the Metrics UI) implicitly use the default source configuration. This works because...

1. there are defaults for all values even no user saved any source configuration before and
2. there is no ambiguity because there is only one per space.

When we introduce a way to manage and switch between multiple source configurations we need to decide what the user will be shown when visiting the Logs UI. Among the possible variants are:

1. Always explicit choice: When the user visits the Logs UI they always have to choose which source configuration to show. When no source configuration exists they have to create one first. Subsequent changes to the source configuration are unambiguously written to the existing source configuration.
2. Explicit choice on first visit: As variant 1 above, but the id of the last visited source configuration is stored locally and restored when the user visits again. An explicit selection of a source configuration is therefore only needed for the first visit or when the user intends to switch to a different source configuration. This is similar to the Kibana dashboard workflow.
3. Implicit choice: As variant 2 above, but without the need for the user to select a source configuration even on first visit. This adds the requirement that there always has to be a "default" source configuration that is chosen when no explicit choice is made or no previous choice can be restored. It is unclear how subsequent changes to such an implicit "default" source configuration are persisted. This interacts with "transient or persistent" architecture choice presented further below.

[weltenwort]

merged into #120932