Create external table over google sheets

### Describe the feature

Bigquery has functionality to create an external table over a google sheet. Dbt-external-tables should include ability to create these external tables.

```
sources:
- name: gsheets
tables:
- name: my_table
description: "External table google sheet"
columns:
- name: my_string
data_type: string
- name: my_bool
data_type: boolean
- name: my_integer
data_type: int64
- name: my_float
data_type: float64
external:
options:
format: google_sheet
sheet_range: MY_NAMED_RANGE
skip_leading_rows: 1
uri: https://docs.google.com/spreadsheets/d/1SywQKI30ookqnfsp

```

Create the external table with:
```
dbt run-operation stage_external_sources \
—args “select: gsheets”
```
As

```
create or replace external table `gcp-project.gsheets.my_table`
(
my_string: string
my_bool boolean
my_integer int64
my_float float64
)
options(
description= ‘External table google sheet’,
format=‘’GOOGLE_SHEETS’,
sheet_range=MY_NAMED_RANGE,
skip_leading_rows=1,
uris=[‘https://docs.google.com/spreadsheets/d/1SywQKI30ookqnfsp’]
)
```

### Describe alternatives you've considered
Common patterns:
- YOLO: I’ll just do it in the UI, we’ll only need to do this once
- orchestrator task

### Additional context
I have working code.

### Who will this benefit?

Sometimes analytics teams can use dbt as the default tool and a context shift to create these external tools is a missed opportunity because it means omitting a small part of the solution from the habits (docs, tests, git) that dbt facilitates. Make it easy to follow good practices.