Include package info example for BigQuery

[ajkannan]

BigQuery is missing a package info example.

The PR that adds this could also fix the weird spacing on the package info examples in general (i.e. at the bottom of the storage javadoc).

[mziccard]

By "weird spacing" you mean the fact that the first line in the snippet seems to be less indented than the other ones? I already struggled a bit to fix that but couldn't come up with a solution that used just one @code.

[ajkannan]

I think if we add a space after the <pre> in the package info lines that say <pre>{@code, we can get the one-character indent that matches the other lines.

Edit: it looks like Datastore's package info did this (see here), so only BigQuery, Storage, and ResourceManager will have to be fixed.

[mziccard]

It seems to me that the space after <pre> still does not make the indentation match but it's much better!