Allow to use custom URLs in elastic-package status

[mrodm]

Part of #2993.

Includes two new configuration into ~/.elastic-package/config.yml to allow overriding the defaults URLs used by elastic-package when executing elastic-package status.
Example:

stack:
    image_ref_overrides: {}
profile:
    current: default

## New configuration fields
status:
  package_registry:
    base_url: "https://epr.elastic.co"
  kibana_repository:
    base_url: "https://raw.githubusercontent.com/elastic/kibana"

Default values will keep the same:

• Package Registry: https://epr.elastic.co
• Kibana URL: https://raw.githubusercontent.com/elastic/kibana

Author's checklist

• Test with custom package-registry and kibana URLs in elastic-package status.
• Ensure that elastic-package keeps using the default URLs if they are not specified in the config.
• Update documentation

How to test this PR locally

• Edit ~/.elastic-package/config.yml including:

status:
  package_registry:
    base_url: "https://epr-staging.elastic.co"
  kibana_repository:
    base_url: "https://raw.githubusercontent.com/mrodm/kibana"

• Run elastic-package status with different parameters:

go run github.com/elastic/elastic-package -C test/packages/parallel/nginx status -vv
go run github.com/elastic/elastic-package -C test/packages/parallel/nginx status -vv --info kibana.version,serverless.project_types

[elasticmachine]

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cc @mrodm