BigQuery: add destinationTableProperties to LoadJobConfig

[yan-hic]

To catch up with https://issuetracker.google.com/issues/70117381

[tseaver]

@tswast while working on this, I am adding unit tests for each of the LoadJobConfig properties. I notice that skipLeadingRows is mangled to a string in the resource, while maxBadRows is not. Which is correct? Given that we are using JSON, and the numbers expected wouldn't be huge, I would think that leaving them both as integers would be correct. The docs show them both as integers.

[tswast]

Per the discovery document (https://www.googleapis.com/discovery/v1/apis/bigquery/v2/rest)

maxBadRows is an int32 and can safely be represented as an integer in Google's JSON parser.

skipLeadingRows is an int64, so needs to be encoded as a string for Google's JSON parser.

I asked the BigQuery team this same question (why the inconsistency in types) and they didn't have a satisfactory answer.