Add samples for data catalog lookup_entry.

[tswast]

These samples use the sample generator to call LookupEntry linked to a BigQuery resource.

Depends on internal CL 246050183 to add the GAPIC config for these samples.

[tseaver]

ISTM that sample code should not be in the main source tree: our normal convention is to place them in datacatalog/docs/samples.py (or perhaps multiple files there).

[tswast]

GitHub doesn't make it easy to see, but the path is samples/google/cloud/datacatalog_v1beta1/gapic/lookup_entry/datacatalog_lookup_entry_by_sql.py. I've complained about the ridiculously long path names to the generator team, though I haven't filed a bug on it yet.

[tseaver]

@tswast I'm not talking about the source from which the sample is generated, but where we put it in this repo: samples for docs should be in the datacatalog/docs subdirectory, not in google/cloud/datacatalog_v1beta1/gapic/: they aren't part of the library, but rather its docs.

[tswast]

Right, I'm saying it's not in google/, it's in samples/. That's just where the generator sticks it. I could work around that by adding move rules to synth.py, but if we do that, we should notify the generator team about why samples/ doesn't work for us. (IMO, samples/ seems appropriate)

CC @beccasaurus

[tseaver]

@tswast I noticed just now that BigQuery has a samples subdirectory containing similarly-generated (I think?) samples / snippets. Could we do that for datacatalog, rather than leaving them inside google/cloud?

[tswast]

Obsolete by GoogleCloudPlatform/python-docs-samples#2148.