[FEA] Support multiple joins at once

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

Pandas supports passing multiple dataframes to `join` at once, cudf does not.

ref: https://pandas.pydata.org/pandas-docs/stable/user_guide/merging.html#joining-multiple-dataframes

**Describe the solution you'd like**

To be able to join multiple dataframes efficiently in one call:
```
result = left.join([right, right2])
```

**Describe alternatives you've considered**

Can currently use `cudf.concat` in some cases:
```
df_wide = cudf.concat([...], axis=1)
```
however `concat` does not accept a `join` keyword to specify the kind of join to perform.