feat(bigquery): load jobs with dest tables in another project

[alvarowolfx]

This PR favors the usage of the dataset.project_id method instead of @service.project to allow Load jobs to target tables in a dataset that lives in a separated project, other than the project usage on the main client and which is used for billing.

A follow up PR needs to be address more scenarios where we should favor that same usage of the dataset.project_id instead of @service.project, since is a common use case where operations are run in project A to modify data stored in project B.

I haven't added integration tests, since we would not have write access to two different projects in our CI pipelines. But I tested locally with this given code:

# $ export GOOGLE_CLOUD_PROJECT=projectA

require "google/cloud/bigquery"

bigquery = Google::Cloud::Bigquery.new
project_id = "projectB"

dataset = bigquery.dataset dataset_id, project_id: project_id # <- this is the important change
gcs_uri  = "gs://cloud-samples-data/bigquery/us-states/us-states.csv"
table_id = "us_states"

load_job = dataset.load_job table_id, gcs_uri, skip_leading: 1 do |schema|
  schema.string "name"
  schema.string "post_abbr"
end
puts "Starting job #{load_job.job_id}"

load_job.wait_until_done! # Waits for table load to complete.
puts "Job finished."

table = dataset.table table_id
puts "Loaded #{table.rows_count} rows to table #{table.id}"

Fixes #27368

[alvarowolfx]

blocked by #27684

[dazuma]

This looks good. Thanks much!