[java-cloud-bom] Add missing dependencies used in java-docs-samples

The following libraries are still referenced by version in [GoogleCloudPlatform/java-docs-samples](https://github.com/GoogleCloudPlatform/java-docs-samples).

The request is to add any of these that would be appropriate to `libraries-bom`. Note that there may be other missing artifacts in the same family, e.g. Spanner, that may also belong, but aren't referenced in the sample repo.

**com.google.cloud**
```
com.google.cloud:google-cloudevent-types
com.google.cloud:cloud-spanner-r2dbc
com.google.cloud:cloud-spanner-spring-data-r2dbc
```

**com.google.cloud.***
```
com.google.cloud.bigtable:bigtable-hbase-1.x
com.google.cloud.bigtable:bigtable-hbase-beam

com.google.cloud.functions:functions-framework-api
com.google.cloud.functions.invoker:java-function-invoker

com.google.cloud.sql:cloud-sql-connector-r2dbc-mysql
com.google.cloud.sql:cloud-sql-connector-r2dbc-postgres
com.google.cloud.sql:cloud-sql-connector-jdbc-sqlserver
com.google.cloud.sql:mysql-socket-factory-connector-j-6
com.google.cloud.sql:mysql-socket-factory-connector-j-8
com.google.cloud.sql:postgres-socket-factory
```