NGSI-LD Validate nested properties dynamically

[Maghnie]

Is your feature request related to a problem? Please describe.
Currently, only the first level of properties is validated. Validating further sublevels should be handled carefully because the number of property sublevels isn't known in advance.

Furthermore, properties can be singular or a list. But right now there's no explicit validation for these data types.

Describe the solution you'd like
Validate properties recursively, until all sublevels are exhausted. Implementing an artificial stopping condition could be useful to prevent the program from running forever, in case of unexpected input.

Also, check if the data type confirms to either a singular property or a list of properties.

Hint: To the functionality, this test entity can be used

FiLiP/tests/models/test_ngsi_ld_context.py

Lines 136 to 147 in 317cfb5

	self.entity_sub_props_dict = {
	"id": "urn:ngsi-ld:Vehicle:test1243",
	"type": "Vehicle",
	"prop1": {
	"type": "Property",
	"value": 1,
	"sub_property": {
	"type": "Property",
	"value": 10,
	"sub_sub_property": {
	"type": "Property",
	"value": 100

Describe alternatives you've considered
Limit the validation to the outermost level of properties and log a message/warning that no further sublevels are validated.

[Maghnie]

Comments continued on the PR page: #320