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

[comphead]

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 #23316 (comment)

[ByteBaker]

@comphead thanks for creating this and tracking the proposal. Let's see how we can proceed on this. I'd like to take this up once we have some clarity on how to go about this.

[comphead]

Thanks @ByteBaker for being interested in this. As @alamb correctly mentioned the RuntimeConfig values shared across session, so we need to investigate if it makes sense RuntimeConfig configs absorbed by SessionConfig(unlikely IMO). Or keep RuntimeConfig is a separate field in SessionConfig but wrapping values into ConfigOptions which would unify framework working with config values