feat(apps): Add Generate App Manifest functionality

[daviddmd]

The Cloud Foundry V3 API reference specifies a manifest endpoint for apps that returns the computed app manifest YAML (string), matching the cloudfoundry-application-manifest.json schema.

I've added the get_manifest method to the AppManager class of the cloudfoundry_client/v3/apps.py module that consumes this API endpoint.

Example:

manifest: str = client.v3.apps.get_manifest(application_guid="97bcba72-43f2-4197-8136-fe178c22846f")
manifest_dict: dict = yaml.safe_load(manifest)

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I've also introduced a fix to the _append_encoded_parameter function of the _get_url_filtered method of the entities module of the networking package, in order to append the GET parameters with the %s=%s format, in line with what the Policy Server API interface expects for its GET requests.

Example:

GET /networking/v1/external/policies
GET /networking/v1/external/policies?id=97bcba72-43f2-4197-8136-fe178c22846f
GET /networking/v1/external/policies?dest_id=8e05d3db-bebf-4373-a9f7-c68686aea169

As of now, the following doesn't work as intended:

client.networking_v1_external.policies.list(**{"id":"97bcba72-43f2-4197-8136-fe178c22846f"})

Expected: https://<cf-instance>/networking/v1/external/policies?id=97bcba72-43f2-4197-8136-fe178c22846f

Actual: https://<cf-instance>/networking/v1/external/policies?q=id%3A97bcba72-43f2-4197-8136-fe178c22846f, which yields all network policies present in the CF instance.