[API]: Unify Send API

Currently each Astarte Device SDKs has a different name for the function that allows publishing data to an Astarte instance.

We want to unify the Send API across all SDKs to avoid confusion for the end user, who may use different SDKs for various use cases and could become disoriented.

All SDKs will implement the unified API for publishing data to an Astarte instance.

API for publishing a property value :

``` python
def set_property(...);

def unset_property(...);
```

API for publishing a individual value :

``` python
def send_individual(...);
```

API for publishing a object value :

``` python
def send_object(...);
```

Start by locating the current publishing methods in the Python SDK and compare them with the proposed set_property, unset_property, send_individual, and send_object API. Check how each existing operation is exposed and tested. Done means the Python SDK implements the unified publishing API consistently, with its tests updated to cover the new method names.

Written by the indexing model from the issue text.