Add default LoadJobConfig to Client

[chelsea-lin]

Is your feature request related to a problem? Please describe.

Client can supports the default QueryJobConfig (commit d7a9a80) a while ago. This is helpful to bind a session_id for the query jobs (related #1512).

bqclient.default_query_job_config = bigquery.QueryJobConfig(
      connection_properties=[
          bigquery.ConnectionProperty("session_id", session_id)
      ]
)

After BQ load jobs support connection_properties with session_id (commit cd0aaa1), we would like to introduce default_load_job_config to Client so that the similar connection properties can be passed.

Describe the solution you'd like

Can we introduce default_load_job_config to Client (similar to commit cd0aaa1)

Describe alternatives you've considered
The alternatives solution would patching the connection_properties to all load_* APIs, which is not workable if we need to call an external libraries (e.g. ibis), where we can only config the client settings but its load APIs are hidden within the external libraries.