[CT-958] Refactor incremental materialization, using new incremental_strategy foundations

[jtcohen6]

Follow up to dbt-labs/dbt-bigquery#223, which implemented the minimal version to keep the current code working

See dbt-labs/dbt-snowflake#196 for reference implementation:

• Define valid_incremental_strategies
• Add to materialization code:

    {#-- Get the incremental_strategy, the macro to use for the strategy, and build the sql --#}
    {% set incremental_strategy = config.get('incremental_strategy') or 'default' %}
    {% set incremental_predicates = config.get('incremental_predicates', none) %}
    {% set strategy_sql_macro_func = adapter.get_incremental_strategy_macro(context, incremental_strategy) %}
    {% set strategy_arg_dict = ({'target_relation': target_relation, 'temp_relation': tmp_relation, 'unique_key': unique_key, 'dest_columns': dest_columns, 'predicates': incremental_predicates }) %}
    {% set build_sql = strategy_sql_macro_func(strategy_arg_dict) %}

• Take stock of remaining discrepancies between default + bigquery incremental materializations, identify opportunities for further refactor + eventual deduplication

[github-actions]

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[dbeatty10]

As-of v1.7, dbt-bigquery currently only supports these incremental strategies:

• merge
• insert_overwrite

So it currently does not support these two:

• append
• delete+insert

https://docs.getdbt.com/docs/build/incremental-models#supported-incremental-strategies-by-adapter

Since dbt-core dbt-adapters has default definitions of append and delete+insert, it should be trivial for dbt-bigquery to support both of those during implementation of this issue.