[CT-634] Node configs: tech debt

Context

Previous version of similar proposal: https://github.com/dbt-labs/dbt-core/issues/3557
Prompted most recently by https://github.com/dbt-labs/dbt-core/pull/5230, which adds grants as a config under NodeConfig, where it gets inherited by node types that won't actually use it.

Problem

We need to better organize our tightly bundled NodeConfig. Many of these configs are only relevant to "executable" nodes (models, snapshots, seeds, and tests IFF store_failures is enabled). Some of them are only relevant to incremental models in particular. Analyses have no business with database/schema/alias.

Existing proposals

Incremental models. Many many configs are really just relevant to the incremental model materialization. As discussed in #5089: It feels a little icky that configs like unique_key and on_schema_change appear in the config for every node, when they're really just relevant to incremental models in particular. Could we isolate these to appear on incremental models only? This could be:

an IncrementalModelConfig subclass, which would supply unique_key, on_schema_change, incremental_strategy, and (new) incremental_predicates if and only if materialized == 'incremental'
a dictionary named incremental that contains strategy + predicates, and the old names can be passthroughs to that dictionary for backwards compatibility

Metadata interface. Every time we add a new attribute to NodeConfig, it changes the manifest.json artifact schema. As discussed in https://github.com/dbt-labs/dbt-core/issues/4617: Given that users can supply whatever configs they want, to any node type that accepts config, should we even consider this a contracted metadata interface? I think the external contract for node.config should really be Dict[str, Any], and we should always be in the habit of "promoting" configs that are relevant and reliable to downstream metadata consumers as top-level node keys. Similar to what we did with node_info on logging events, which "promotes" config.materialized into node_info.materialized.

Adapter-specific configs (sort of discussed in https://github.com/dbt-labs/dbt-core/issues/4622): I'm actually not sure if AdapterConfig works at all today. My sense is that their keys don't show up in the manifest unless set, their types aren't really validated until used (at runtime), and their default values don't get passed through. The code here is kinda jank, to put it nicely.

dbt-labs / dbt-core