crosscluster: add crdb_route parameter for LDR and PCR

[jeffswenson]

The crdb_route query parameter determines how the destination
cluster's stream processor connects to the source cluster. There are two
options for the query parameter: "node" and "gateway". Here is an
example of using the route paraemeter to create an external connection
that is usable for LDR or PCR.

-- A connection that routes all replication traffic via the configured
-- connection URI.
CREATE EXTERNAL CONNECTION 'external://source-db' AS
'postgresql://user:[email protected]:26257/sslmode=verify-full&crdb_route=gateway'

-- A connection that enumerates nodes in the source cluster and connects
-- directly to nodes.
CREATE EXTERNAL CONNECTION 'external://source-db' AS
'postgresql://user:[email protected]:26257/sslmode=verify-full&crdb_route=node'

The "node" option is the original and default behavior. The "node"
option requires the source and destination clusters to be in the same IP
network. The way it works is the connection string supplied to LDR and
PCR is used to connect to the source cluster and generate a physical sql
plan for the replication. The physical plan includes the
--sql-addvertise-addr for nodes in the source cluster and processors
in the destination cluster connect directly to the nodes. Using the
"node" routing is ideal because there are no extra network hops and the
source cluster can control how load is distributed across its nodes.

The "gateway" option is a new option that is introduced in order to
support routing PCR and LDR over a load balancer. When specified, the
destination cluster ignores the node addresses returned by the physical
plan and instead opens a connection for each processor to the URI
supplied by the user. This introduces an extra network hop and does not
distribute load as evenly, but it works in deployments where the source
cluster is only reachable over a load balancer.

Routing over a load balancer only requires changing the destination
clusters behavior. Nodes in the source cluster were always implemented to
act as a gateway and serve rangefeeds that are backed by data stored on
different nodes. This support exists so that the cross cluster
replication does not need to re-plan every time a range moves to a
different node.

Release note (sql change): LDR and PCR may use the crdb_route=gateway
query option to route the replication streams over a load balancer.

Epic: CRDB-40896

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[cockroach-teamcity]

This change is 

[jeffswenson]

This change depends on #138689 and #138689