Samples should use a single credential

In all the Jupyter notebook samples, there's a line:

client = FeathrClient(config_path=tmp.name)

However, if the users have configured credential in the previous line, like this:

credential = DefaultAzureCredential(exclude_interactive_browser_credential=False)

this `credential` variable isn't passed to `FeathrClient`, which causes trouble because FeathrClient will initialize a credential again which might be different.

We should change all the lines to below to use the same credential across the sample notebook.

client = FeathrClient(config_path=tmp.name, credential=credential)