More Variables available to -exec flag

**Is your feature request related to a problem? Please describe.**
When credential injection works, it's really awesome. However, it has a very limited feature set right now (namely, SSH). It would be cool to be able to inject credentials on more connections

**Describe the solution you'd like**
Until Boundary gets native support for credential injection for other apps, it might be cool to get brokered credentials in a set of variables you can pass to -exec along with the connection info you presently get. Then, you could run any client program you want that accepts credentials on the command line, or execute any script with those variables as arguments to do whatever you want.

Alternately, if there was a way to write local connection helpers, that would be cool too as long as the required info (host,port,user,pass) are available to the helper. Maybe like #2454 which would also solve the problem

**Describe alternatives you've considered**
I'm considering writing a script that brokers the credentials for me from Vault instead of asking boundary to do it. This would work, I think, but isn't great UX.

**Explain any additional use-cases**
Any use case where I'm trying to get simulate credential injection in lieu of boundary natively supporting it.

**Additional context**
Add any other context or screenshots about the feature request here.