Feature/Add-append-materialization-macro-for-snowflake

[gsamp001]

dbt's snowflake profile has doesn't have the ability to materialize incremental models under a strictly append context. It can only materialize incremental models under the merge context.

this becomes an issue when working with large incremental models whose data is unchanging in nature. Example: an events table in snowflake.

the computational cost of appending new entries AND searching for changes within already entered events is always larger than the cost of appending new rows if we create an append macro 👍