Support an extention path to parameter store to swap secrets/vars under the same project

[daigotanaka]

Currently, a secret <some_key> is stored in the remote parameter store (AWS SystemsManager Parameter Store) in the following format:

/<resource_group_name>/<task_name>/<some_key>

To make the project more reusable, the secrets keys can use an extension like this:

/<resource_group_name>/<task_name>/<some_key>/__ext/<some_extension>

The value is still loaded to <some_key> although it is retrieved from a key ending <some_extension>.
A reserved key __ext is added to handle the separation between the extension and the rest. This is necessary for handling an extension for resource group-level secrets.

Example 1:
This extension can be a notion of production or staging environment.

/<resource_group_name>/<task_name>/<some_key>/__ext/staging

Example 2:
This extension can be used to give the same task definition execute with different nuances of contexts:

/<resource_group_name>/<task_name>/<some_key>/__ext/account_1
/<resource_group_name>/<task_name>/<some_key>/__ext/account_2

handoff commands can take --extension (shorthand -x) option like:

# Local run
handoff run local -p <project_dir> -w <workspace_dir> --extension account_1

# Local run with remote stored secrets and project settings
handoff run -p <project_dir> -w <workspace_dir> --extension account_1

# Local container run test
handoff container run -p <project_dir> -w <workspace_dir> --extension account_1

# Run on cloud
handoff cloud run -p <project_dir> -w <workspace_dir> --extension account_1

With run local, handoff should search a subdirectory .secret/account_1 instead of the default directory .secret.