PolicyAdd rpc replaced by PolicyAddService & PolicyAddDevice

- restructured component and separated it into "connector" and "gateway" subcomponents.
- arranged name typos in DLT proto file
- prepared component folders for dlt-connector component

- Defined new dlt.proto file

Updated gRPC proto files to prevent trouble with KPI sample types in devices, also updated files related to this issue.

Deactivated some Device monitoring functionality partially implemented to enable a functional commit into develop.

Device:
- Implemented Driver API with some special resources to enable retrieving endpoints from devices, interfaces, and network instances.
- Implemented special resource_keys (_connect/*) to enable AddDevice to obtain settings used to connect to remote devices (address, port, username, password, etc.)
- Implemented Emulated Driver (only for testing)
- Implemented OpenConfig Driver (missing to implement monitoring; templates for Infinera devices to be tested with other device vendors)
- Minor bugs and code improvements
- Improved test unit to enable easy testing of vendor devices and keep credentials secret.

Policy component's protobufs are updated based upon
the Event Condition Action (ECA) model.
Minor fixes are applied to the slice.proto in order
to compile.

Signed-off-by: Georgios Katsikas <katsikas.gp@gmail.com>

- Corrections in Service to prevent errors in GitLab CI/CD pipeline.
- Minor improvements in random classes.

Common:
- added ORM helper function get_related_instances
- minor modifications in ORM framework

Proto:
- field/message/enum renamings in context.proto to normalize notation

Context:
- Expanded service/database/Models.py into separate models to facilitate coding
- Completed implementation of Get/Set/List/Remove methods for Context, Topology, Device, Link, and EndPoint; also completed for relation models between topology and device/link, and ConfigModels
- Implemented test units for Get/Set/List/Remove methods for Context, Topology, Device, Link, and EndPoint
- Implemented skeleton for Get/Set/List/Remove methods for Service.
- Added Tools.py with helper methods to fimplify handling of enums between ORM and gRPC messages.

Proto:
- Minor corrections in context.proto

Common:
- Minor corrections in common.orm.backend
- Improved definition of orm.Model class with get_instances and get_references
- Extended ORM unitary tests with get_instances and get_references

Context:
- Implemented missing methods in ContextClient
- Started to implement new Context data models using ORM.
- ContextModel functional using ORM and gRPC.
- Arranged old code file placing

- Context Database API and ORM rework in progress

- Complete rework of Database API
- Integration of Database API into Context
- Improvement of API

This commit is not a final implementation, just an intermediate backup.

Fix renaming of compute.proto methods from snake-case to camel-case. Added minor naming improvements.

Common:
- minor bug resolutions in Context API

Service:
- first functional version of Service (implementation of GetConnectionList postponed)
- minor corrections in service.proto and related files
- implemented unitary tests

Common:
- Improvements in Database API definition and unit tests
- Implemented additional Assertions to be used by unitary tests

Context and Device:
- Improved logging of errors.

Service:
- Implemented service methods (except GetConnectionList). All methods are under active debug.
- Partially implemented unitary tests