[Feature] Enable user to provide custom adapter via profiles.yml

[ismailsimsek]

resolves dbt-labs/dbt-core#3962

Problem

Currently customizing dbt adapters and dbt functionalities is very hard to not possible. This PR enables end user to extend exsting adapters and add his/her own customization(special needs) to to it

Solution

Enabling end user to provide his/her custom adapter, using the adapter_class setting in profiles.yml

snowflake:
  target: dev
  outputs:
    dev:
      type: snowflake
      adapter_class: my.customized.adapter.SnowflakeAdapterV2Customized

and user adapter could be something like

class SnowflakeAdapterV2Customized(SnowflakeAdapter):
    def __init__(self, config) -> None:
        print(f"WARNING: Using User Provided DBT Adapter: {type(self).__module__}.{type(self).__name__}")
        super().__init__(config=config)

    @available
    def my_custom_method(self, arg1: str, arg2: str):
        print('do something here')

Checklist

• I have read the contributing guide and understand what's expected of me
• 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
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX

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[ismailsimsek]

cc @jtcohen6 could you please have look? I believe the changes gives the idea. if it looks good then i will ad test as well

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[dbeatty10]

Like @jtcohen6 explained in dbt-labs/dbt-core#3962 (comment), it's been a deliberate decision to not allow user-space Python code within your dbt project. We see this as serving as an important security and complexity safeguard, and we do not plan to change it. So I'm closing this PR accordingly.