Add persistent storage grpc API

Add persistent storage grpc API

In the current demo, user settings such as radio stations and HVAC preferences are lost upon poweroff. The agl-persistent-storage-api
adds a simple grpc API that enables persistent storage of
key-value pairs using RocksDB.

The API supports Write, Read, Delete, and Search calls, as well as
calls to recursively list or delete nodes (intended for use with
VSS-like keys) and to destroy the database. Separate namespaces
are also supported.

The API is written in Rust, the repo can be viewed under this URL:
https://github.com/LSchwiedrzik/agl-persistent-storage-api

Test builds were successfully carried out by Jan-Simon Möller.

Bug-AGL: [SPEC-5187]
Change-Id: Ic11d31c75268f922c84785da8cd29fa6b022612f
Signed-off-by: Ludwig Schwiedrzik <ludwig.schwiedrzik@d-fine.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/29996
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jenkins Job builder account