Resolve initial_value for incremental from config

[sultaniman]

This PR will address the issue #1034 where it is not possible to fetch initial_value from config.toml

@dlt.resource(write_disposition="merge", table_name="referrals", primary_key="id")
def referrals_resource(
    api_secret_key=dlt.secrets.value,
    createdAfter=dlt.sources.incremental(
        "created",
        # BUG: cannot read this initial_value from config file
        initial_value=dlt.config.value,
        last_value_func=last_value_func,
    ),
    campaign_ids=dlt.config.value,
    schema_contract=SCHEMA_CONTRACT
):

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[rudolfix]

@sultaniman as discussed - this needs docs update where we show how to configure incremental. so close this or repurpose the PR

[sultaniman]

@rudolfix thanks for reminding it, I created a documentation ticket for this #1077, also @dat-a-man has a PR #1059 I will likely prepare an update for his PR and send it over.