Snapshot arguments/function

[muschellij2]

We want to make snapshots/backups of our tables, which is available via: https://cloud.google.com/bigquery/docs/table-snapshots-create#api.

Namely, adding in something about operationType = "SNAPSHOT" in docs for bq_perform_copy (arg name operation_type) and likely destinationExpirationTime. Would this operation be useful to put in bigrquery?

If so, I'm not sure the best course of action, but the options are:

1. Added documentation of arguments (in ...) for bq_table_copy
2. Additional arguments for bq_table_copy (may cause confusion), but we can map all the options from https://cloud.google.com/bigquery/docs/reference/rest/v2/Job#JobConfigurationTableCopy, with default COPY
3. Another function for snapshotting, that calls bq_table_copy (or tell users to do that.

I'd be happy to discuss these options, but wanted to gauge interest before doing any work on a PR.

[hadley]

If you're still interested, this still seems worth working on.