Rework _dbt_max_partition

[jtcohen6]

resolves #17
resolves #39

Summary of changes

• Rather than hard-coding the declaration of _dbt_max_partition in the dynamic insert-overwrite script, shell out to a macro declare_dbt_max_partition() that users can override with custom logic
• Keep the default logic simple (+ expensive) for now, but only declare _dbt_max_partition if _dbt_max_partition is present in the compiled SQL
• That custom logic could include:
  • disabling this variable entirely
  • using get_partitions_metadata (i.e. querying $__PARTITIONS_SUMMARY__ with legacySQL)
  • using INFORMATION_SCHEMA.PARTITIONS (standardSQL)
• Include _dbt_max_partition declaration alongside temp table creation, as needed for on_schema_change. Add an integration test for this. (Should we try to be fancier about this, and actually append _dbt_max_partition to the sql_header config? I'd prefer to keep this simple + janky for now.)

Checklist

• 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 updated the CHANGELOG.md and added information about my change to the "dbt-bigquery next" section.

[alexras]

LGTM