Decouple tedge-mapper-c8y and tedge-agent for config_update

[rina23q]

Is your feature improvement request related to a problem? Please describe.
The config_update feature requires downloading files from cloud, then storing it to <data_dir>/cache directory (e.g. /var/tedge/cache). The cache feature is very helpful to save bandwidth because it can prevent downloading the same files multiple times from the internet.

As of now, upon receiving a config_update operation from c8y, tedge-mapper-c8y

• checks if a file associated with remoteUrl is available in the cache directory
• downloads a file if not available in the cache directory and stores the file to there

The problem is, accessing to <data_dir>/cache is the responsibility of tedge-agent. So, tedge-mapper-c8y should not access the directory directly. (For the background of decoupling, refer to #2390)

Describe the solution you'd like
The solution is not yet very clear. The must is that tedge-agent downloads a file from cloud.

Describe alternatives you've considered

Additional context

[didier-wenzek]

See #2071 (comment)

A solution could be to add a cache method in the file-transfer service. A POST request asking the tedge-agent to download a file from the cloud and to serve it locally.

[Bravo555]

QA instructions

The modified test at tests/RobotFramework/tests/cumulocity/configuration/configuration_with_file_transfer_https.robot should cover the changes. The gist is that tedge-agent should now be fully functional when run on a separate device/container. The new device/container still has to be configured as the main device (via mqtt.device_topic_id) and http.client.host/port settings should be set only on that physical device on which the agent is running, so that a correct tedgeUrl is generated in operation message payloads.

[gligorisaev]

QA has thoroughly checked the feature and here are the results:

• Test for ticket exists in the test suite.
• tests/RobotFramework/tests/cumulocity/configuration/configuration_with_file_transfer_https.robot
• QA has tested the function and it's functioning according description.