Add subscription argument to SRE config

[jemrobinson]

:white_check_mark: Checklist

• [x] I have searched open and closed issues for duplicates.
• [x] This is a request for a new feature in the Data Safe Haven or an upgrade to an existing feature.
• [x] The feature is still missing in the latest version.
• [x] I have read through the documentation.
• [x] This isn't an open-ended question (open a discussion if it is).

:strawberry: Suggested change

Currently SREs don't have their own subscription argument and will be deployed into the same subscription as the SHM and Context. We should make this user-configurable.

See also #1633.

:steam_locomotive: How could this be done?

Add a subscription name/ID option to the SRE config and ensure this is enforced at deployment time.