Initialize lift + shift for cross-db macros

[jtcohen6]

a step on the way toward resolving #4813

Description

• Start with dateadd + datediff
• Move dispatched + default__ macros into global project, macros/utils
• Move Postgres-specific implementations into dbt-postgres
• Move test cases into dbt-tests-adapter, with options to override the macro_namespace. This allows us to test for backwards compatibility when people call the dbt_utils-namespaced versions.

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 — TK
• I have run changie new to create a changelog entry

[jtcohen6]

Superseded by #5298