FR: a way to have variables available to flows\nodes without the value being visible as text

### Description

Use case:
we have different people working on flows, for some of which they need access to resource credentials. For some credentials it is not desirable to have these generally available.
For this, a system like in Powershell where you can store credentials in an object and save and recall that object for later or other use and the password is protected, would be helpful.

Difficulty here is that the people working on flows are (and need to be) FF Admins as well. So there would need to be a kind of system where an admin could define credentials or a string, (for API keys or bearer tokens, or just a password) which can then be used by all admins in a team in flows. This "protected" string would then not ever reveal its content in a user interface (debug messages, on screen dashboards, etc,).

Also see:
https://discourse.nodered.org/t/managing-secrets-in-flowfuse/87786/10

### Which customers would this be available to

Enterprise Tier Only (EE)

### Have you provided an initial effort estimate for this issue?

I am not a FlowFuse team member

No implementation files, entry points, or tests are identified. Start by reading the issue and the linked FlowFuse discussion, then establish the security and Enterprise requirements for shared protected values. Done would require an agreed design and acceptance criteria ensuring credentials, API keys, and passwords are not exposed through interfaces, debug output, or dashboards.

Written by the indexing model from the issue text.