[Feature] Change BigQuery `labels` to be additive instead of "clobber"

### Current Behavior

When we set labels in `dbt_project.yml`
```
finance:
+labels:
domain: finance
test: "yes"
```

and then extra ones in the model file

```
{{
config(
labels = {"type": "financial", "contains_pii": "yes"}
)
}}
```

The configuration of the model gets overwritten, even though the labels are different.


### Expected Behavior

I expected all labels to be applied and if a label is specified twice then for the single model configuration to take precedence.

### Steps To Reproduce

Create a sample model in folder `models/finance/finance_test.sql` and add the following configuration:

```
{{
config(
labels = {"type": "financial", "contains_pii": "yes"}
)
}}
```

In `dbt_project.yml` add the following configuration:

```
models:
project_name:
finance:
+labels:
domain: finance
test: yes
```

### Relevant log output

_No response_

### Environment

```markdown
- OS: Windows 10
- Python: 3.8.4rc1
- dbt: 1.7.7
- dbt-bigquery: 1.7.4
```

### Additional Context

_No response_