Load balancing connections among nodes of RR cluster.

[HarshDaryani896]

IDEA: IDEA-1330

Implementation details: Doc

The connection property load_balance currently allows only two values, viz. false (the default) and true.

It will now also allow below five new values to be specified:
only-rr - Create connections only on Read Replica nodes
only-primary- Create connections only on primary cluster nodes
prefer-rr - Create connections on Read Replica nodes. If none available, on any node in the cluster including primary cluster nodes
prefer-primary - Create connections on primary cluster nodes. If none available, on any node in the cluster including Read Replica nodes
any - Equivalent to value true. Create connections on any node in the primary or Read Replica cluster

default value is still false

Other Minor changes:

• Corrected the behaviour of fallback_to_topology_keys_only connection property. When fallback_to_topology_keys_only is set to true and all the nodes of the specified topology_keys are down, the driver currently fallbacks to upstream driver behaviour but the driver is expected to throw an error in this case which is corrected in this PR.
• When validating topology_keys, do not match them with the placement info of the cluster.

Testing:
Test cases identified and listed in this doc.
Test for all test cases listed in the doc added to driver-example repo: PR LINK.
Ran the existing and new tests in driver examples repo for rust-postgres.

[Sfurti-yb]

LGTM