[CT-2790] Support dispatch for materialization macros

[jtcohen6]

Definition of done:

• It's possible to call adapter.dispatch for materialization macros, and return the appropriate macro for this adapter, which can then be called/executed from within another macro/materialization

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In conversation with Kshitij just now, it seems that the main changes need to be in core/dbt/context/macros.py, in MacroNamespace.get_from_package where we can add special processing of "materialization" macros. The "get_from_package" method is called from adapter.dispatch, which comes from the adapter "contextproperty" of the ProviderContext in core/dbt/context/providers.py. The actual "dispatch" method is in the BaseDatabaseWrapper.

The materialization macro can be found in the "execute" method of the clone task (and probably the same method in runnable.py) by doing context.adapter.dispatch from the constructed context. This should already be a MacroGenerator callable, so we wouldn't need to construct a MacroGenerator as we do further down in "execute".

Originally posted by @gshank in #7799 (comment)

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Motivation

This would enable us to refactor the implementation in the clone materialization here:

dbt-core/core/dbt/include/global_project/macros/materializations/models/clone/clone.sql

Lines 49 to 58 in a58b5ee

	-- reuse the view materialization
	-- TODO: support actual dispatch for materialization macros
	-- Tracking ticket: https://github.com/dbt-labs/dbt-core/issues/7799
	{% set search_name = "materialization_view_" ~ adapter.type() %}
	{% if not search_name in context %}
	{% set search_name = "materialization_view_default" %}
	{% endif %}
	{% set materialization_macro = context[search_name] %}
	{% set relations = materialization_macro() %}
	{{ return(relations) }}

It might also unblock an approach for this desirable UX improvement to model versions:

• [CT-2468] [Feature] For versioned models, automatically create view/clone of latest version in unsuffixed database location #7442

[gshank]

This would constitute a new way of calling materialization macros with a different method of inheritance. There's also some complexity in that materialization macros can have parameters that don't exist in "normal" macros which would need to be handled.