Add subscription argument to SRE config

[jemrobinson]

✅ Checklist

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

🍓 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.

🚂 How could this be done?

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