feat: grpc keep alive settings

[surbhigarg92]

This PR sets default grpc keep alive options for Spanner library

'grpc.keepalive_time_ms': 30000,
'grpc.keepalive_timeout_ms': 10000,

These settings can be over written by passing to spanner options

const spanner = new Spanner({ 
    'grpc.keepalive_time_ms': 30000,
    'grpc.keepalive_timeout_ms': 10000,
  });

[olavloite]

I changed the title of the PR to feat: instead of chore:, because I think this should go into the release notes.