Support `spread` for `scatter` and `points`

#### Is your feature request related to a problem? Please describe.

`spread` [appears](https://hvplot.holoviz.org/reference/tabular/scattermatrix.html#:~:text=%3DTrue%20or-,spread%3DTrue,-.%20Head%20over%20to) to be supported with scatter_matrix, but not scatter :(
Dynspread is great, but very often spread is what you need!

#### Describe the solution you'd like
Support `spread` for scatter and points

#### Describe alternatives you've considered
Reducing `pixel_ratio` makes scatter points more visible but also heavily pixelated.

#### Additional context

```python
import pandas as pd
import numpy as np
import hvplot.pandas
df = pd.DataFrame(np.random.randn(100_000, 4), columns=['A','B','C','D'])
df.hvplot.scatter(x='A', y='B', rasterize=True, spread=4)
```

```python
from hvplot.plotting import scatter_matrix
scatter_matrix(df, rasterize=True, spread=4)
```