Implement create sub api

Problem statement:
Create subscription API to apply measurement parameters(KPIs) on network functions requested by participants such that PMSH can accept and process multiple requests.

Background:

• Initially, one subscription was processed with network functions for respective measurement details.
• Accordingly, the subscription details were obtained through CBS by refreshing app configs.
• Therefore, PMSH was capable of handling only single subscription tightly coupled with CBS.

Solution:

• Multiple subscription request can be posted by a participant. Therefore, CBS will no longer provide a subscription.
• A subscription will contain multiple measurement groups to associate with network functions requested by Participant.
• The key fields(file location, granularity period, administrative status) will be moved from subscription to each measurement group(KPIs). Thereby, separately capturing KPI's on network functions for each group in measurement groups.

Specific details:

• Key identifiers in subscription request are subscription name, measurement group name, admin status that cannot be empty.
• Valid network function filters with at least one filter value is required (existing rule).
• Utilise existing unit tested functions in PMSH as much as possible.
• Apply and publish measurement group against network functions as per the policy updates. Basically, the policy published will contain an extra field measurement group name to track the association of network function and measurement group.