feat: cli plugin

[hgiasac]

• Enable the configuration CLI plugin and improve the configuration schema.
• Add concurrency settings.
• Support headers forwarding.
• (Breaking change) Replace env template with an object.

# before
url: "{{FOO_SERVER_URL:-https://onesignal.com/api/v1}}"

# after:
url: 
  env: FOO_SERVER_URL
  value: https://onesignal.com/api/v1

• (Breaking change) Move retry and timeout policies to the connector configuration.

[github-actions]

Package	Line Rate	Health
github.com/hasura/ndc-rest/connector	51%	➖
github.com/hasura/ndc-rest/connector/internal	54%	➖
github.com/hasura/ndc-rest/ndc-rest-schema/configuration	60%	➖
github.com/hasura/ndc-rest/ndc-rest-schema/openapi/internal	51%	➖
github.com/hasura/ndc-rest/ndc-rest-schema/openapi	33%	❌
github.com/hasura/ndc-rest/ndc-rest-schema/schema	36%	❌
github.com/hasura/ndc-rest/ndc-rest-schema/utils	39%	❌
Summary	50% (2743 / 5477)	➖

Minimum allowed line rate is 40%