Tidying up materialization logic: relation names, cache loading, etc

[jtcohen6]

Inspired by:

• @epapineau's awesome work consolidating tricky materialization logic in Truncate relation names when appending a suffix #4921, which I was trying to test / understand locally
• Work we've scoped for v1.2 (Planning for v1.2: "Adapter Ergonomics" #5091), including [CT-505] Streamline incremental materialization + strategies #5089 — with the goal of doing more in core materializations, and require less copy-paste-edited code in adapter plugins

Description

• Rename load_relation to load_cached_relation (with backwards compat). Use this macro much more frequently, rather than adapter.get_relation (verbose + harder to read)
• Even more consistent naming: existing_relation (never old_relation), temp_relation, intermediate_relation, backup_relation. Always X_relation, never X_identifier.
• Use get_create_table_as_sql or get_create_view_as_sql, instead of create_table_as or create_view_as (per Reorganize global project (macros) #4154). Long-term goal: better distinguish between static macros that -> str ("returns SQL"), and "dirty" macros that -> Result (runs query by itself, a la adapter.rename_relation).
• Adds back test case removed in Truncate relation names when appending a suffix #4921, with the understanding that before these PRs go into main, this will need to be ported over to the new test location here

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

[jtcohen6]

Code quality checks are failing because of black <> click incompatibility. We resolved this on main a while ago, it's just coming up here because I branched off #4921. We'll get it fixed when rebasing against main.

[McKnight-42]

This all looks good to me

[jtcohen6]

Hmm not sure why Snyk didn't run. Going to try my other special move, closing and reopening

[jtcohen6]

Thanks for reviewing @McKnight-42! I'm going to merge, since this is really just a rename-style refactor, and all the tests are passing :)