Make Census API key optional and move to advanced parameters for all US2020 Census nodes

[sturmkev]

In the config field "US Census APIkey" the node uses a plain string as a default value. This doesn't work, if the user doesn't change this.
Either we make this string like a gray colored recommendation, which is actually not treated as a string upon execution OR we leave it empty, since the node works that way already.

Additionally, we would need to adapt the resulting error message, which is not giving a proper hint on what might be wrong (see screenshot below).

[koettert]

@koettert check what error message is returned if we don't specify an API key and perform more than 500 requests or request more than 50 variables for details see
https://www.census.gov/content/dam/Census/library/publications/2020/acs/acs_api_handbook_2020_ch02.pdf

[UrbanGISer]

PR submitted