Implement wallet import/export functionality

[esune]

In some scenarios it becomes necessary to export an existing wallet and/or import a wallet created elsewhere.
We need functions to facilitate this behaviour.

Use case examples:

• Export all the tenants from a single database multi-tenant instance and import each tenant into a multi-database equivalent multi-tenant instance. The script can request each tenant, so the export doesn't need to handle the "all" part of that.

• Export a tenant from one multi-tenant instance and import into another multi-tenant instance. Could be single- or multi-database deployment.

• Export a single-tenant wallet and import into a multi-tenant (single database or multi-database) instance (migration to a multitenant agent)