Add support for R2DBC

This commit introduces support for R2DBC ("Reactive Relational Database
Connectivity") with custom ConnectionFactory implementations, a
functional DatabaseClient for SQL execution, transaction management, a
bind marker abstraction database initialization utilities, and
exception translation.

Closes gh-25065

settings.gradle

@@ -24,6 +24,7 @@ include "spring-expression"
 include "spring-instrument"
 include "spring-jcl"
 include "spring-jdbc"
+include "spring-r2dbc"
 include "spring-jms"
 include "spring-messaging"
 include "spring-orm"