chore: Explore combining `SessionConfig` and `RuntimeConfig` into same framework

A followup on #23316

Can we please explore if runtime config can be part of `SessionConfig`? In `SessionConfig` there is already a framework that enforces type checking.

Runtime configs looks looks finite according to `set_runtime_variable`, so should fit into main configs rather than having separate config set

One difference was that the `RuntimeConfig` is potentially shared across multiple `SessionConfigs` (and maybe can also be adjusted during plan execution)

Trying to consolidate `RuntimeConfig` and `SessionConfig` should be considered as an independent project

_Originally posted by @alamb in https://github.com/apache/datafusion/issues/23316#issuecomment-4881517563_

Start by tracing SessionConfig, RuntimeConfig, and set_runtime_variable in the DataFusion source. Compare the existing type-checking framework with runtime configuration, including sharing across multiple SessionConfigs and adjustment during plan execution. Done means documenting whether consolidation is viable and resolving those lifecycle differences in a concrete design.

Written by the indexing model from the issue text.