feat(bigquery): add media options to LoadConfig

[alvarowolfx]

Allow users to customize how media upload happens on BigQuery Load Jobs.

// Load the table from a reader.
r := strings.NewReader("a,0\nb,1\nc,2\n")
wantRows := [][]Value{
	{"a", int64(0)},
	{"b", int64(1)},
	{"c", int64(2)},
}
rs := NewReaderSource(r)

loader := table.LoaderFrom(rs)
loader.WriteDisposition = WriteTruncate
loader.Labels = map[string]string{"test": "go"}
loader.MediaOptions = []googleapi.MediaOption{ // <- New option
	googleapi.ContentType("text/csv"),
	googleapi.ChunkSize(googleapi.MinUploadChunkSize),
}

Towards internal b/301126103