Feature: allow handling a single firewall rule

[ralbertazzi]

The current cloudamqp_security_firewall requires one to track all firewall rules. This however creates a conflicting situation when:

• we'd like some rules - such as the ones related to VPC peering - to stay fixed and be tracked on Terraform
• we'd like some other rules - such as the ones that allow human users to access RabbitMQ from their workstation - to change over time and even be edited manually

During this situation, Terraform will now propose a firewall change every time one manually edits the firewall rules. Considering that one use case of opening a firewall rule is to apply Terraform resources on the internal RabbitMQ instance, this effect is deleterious. We currently decided not to track firewall rules on Terraform because of this.

We'd like a Terraform resource that allows us tracking a single firewall rule. This way we could have both "static" rules (tracked on Terraform) and "dynamic" rules (edited manually) without Terraform creating unneeded diffs.

The server API to achieve this should already be there, aka the PATCH https://api.cloudamqp.com/api/security/firewall

When running terraform plan or apply, it would be nice to fetch all single-rule resources through a single API call to GET https://api.cloudamqp.com/api/security/firewall, even if multiple single-rule resources are defined.

[dentarg]

Sounds resonable to me, seeing the API already allows it

[dentarg]

We have #216 for this

However, we ran into some backend challenges that are yet to be resolved.