Added config-file flag to accept file based input params

[Ganeshrockz]

Why is the change required?

As part of the work needed to migrate consul-ecs to use consul-dataplane and remove consul clients from the ECS tasks, we would need to make the consul ecs control plane configure the dataplane config. Since ECS tasks are immutable by nature, we can't call the dataplane CLI command with the appropriate parameters similar to how it is being done in consul-k8s.

According to the RFC, it was decided that the Consul ECS control plane would take of care of writing the configuration needed to start dataplane to a shared volume and pass the filename as an input parameter to the consul-dataplane command. Dataplane would then configure itself based on the config values present in the file.

What has changed?

This PR introduces an additional string input parameter -config-file which accepts a json file for now. We can later extend it to accepting hcl files if needed.

How is the change implemented?

CLI flag inputs (if provided) still holds the highest precedence even if the same config input field is present the config file. Upon startup, dataplane prepares a default config based on the default values present already for various fields. Following that, dataplane reads the configuration present in the json file, unmarshals the input and prepares a config struct based on the flags present in the file. It then merges the second config onto the first config if the values present in the second config aren't the default ones. Finally dataplane constructs a third struct based on the CLI flag inputs and merges it to the previously formed config. In this way, we can make sure that the right order of precedence is maintained when accepting and processing inputs

How was the change tested?

• Unit tests (covering various input generation flows
• CI

[pglass]

Few other minor comments, but this looks good to me

[pglass]

Why define this as a JSON string that is parsed, instead of defining a DataplaneConfigFlags struct directly?

[Ganeshrockz]

Since the fields of DataplaneConfigFlags are mostly pointers, it kind of looks ugly referencing the const variables inside the struct. We can't define const variables for these default flags and assign them as &DefineConsulFlag to those fields. We would have to fallback to primitive types for these variables which doesn't look that good as well. So I went ahead with a hardcoded json that is easier for people to make changes. I can reiterate on this if people see further concerns.