test(bigtable): Pass reverse flag from proxy to library

[bhshkh]

b/387315971

This PR fixes following conformance tests:

1. TestReadRows_NoRetry_OutOfOrderError_Reverse

=== RUN   TestReadRows_NoRetry_OutOfOrderError_Reverse
    readrows_test.go:168: 
        	Error Trace:	/usr/local/google/home/bahaaiman/Documents/cfdb-workspace-01/cloud-bigtable-clients-test/tests/readrows_test.go:168
        	Error:      	"out of order row key, must be strictly increasing. new key: \"row-01\" prev row: \"row-07\"" does not contain "decreasing"
        	Test:       	TestReadRows_NoRetry_OutOfOrderError_Reverse
    readrows_test.go:169: The full error message is: out of order row key, must be strictly increasing. new key: "row-01" prev row: "row-07"
--- FAIL: TestReadRows_NoRetry_OutOfOrderError_Reverse 

2. TestReadRows_Retry_LastScannedRow_Reverse

=== RUN   TestReadRows_Retry_LastScannedRow_Reverse
    readrows_test.go:349: 
        	Error Trace:	/usr/local/google/home/bahaaiman/Documents/cfdb-workspace-01/cloud-bigtable-clients-test/tests/readrows_test.go:349
        	Error:      	Should be true
        	Test:       	TestReadRows_Retry_LastScannedRow_Reverse
--- FAIL: TestReadRows_Retry_LastScannedRow_Reverse 

Cause:
The proxy does not pass the reverse scan flag when calling the methods of the library

Fix:
Add ReverseScan flag in ReadRows call.

Logs after fix:

=== RUN   TestReadRows_NoRetry_OutOfOrderError_Reverse
    readrows_test.go:169: The full error message is: out of order row key, must be strictly decreasing. new key: "row-07" prev row: "row-03"
--- PASS: TestReadRows_NoRetry_OutOfOrderError_Reverse 
=== RUN   TestReadRows_Retry_LastScannedRow_Reverse
--- PASS: TestReadRows_Retry_LastScannedRow_Reverse