Add example queries to BIGGIM

[erikyao]

Now the "Generate New" button does not work under https://biothings.ncats.io/biggim

Input from Marco:

We have it hardcoded that if a source has more than 250000000 docs it will use hardcoded examples in the metadata itself under a field example_queries
I believe we had an API called FIRE that was huge and caused lag issues because of this

Some hardcoded queries are needed in the API's metadata.