Enabling Credential Injection in Boundary for SSH-OTP secret engine

**Is your feature request related to a problem? Please describe.**
In order to access linux servers through Boundary, we are getting dynamic credentials for these servers using SSH OTPs from Vault. Currently, we are leveraging credential brokering for the same, which means once we connect to the target, the user has to copy the OTP (key field in the secret) and then paste that OTP in the ssh terminal.

**Describe the solution you'd like**
We would like to leverage SSH Credential Injection feature along with this secret engine and it seems that this is currently not supported. Use of this feature along with embedded terminals will give a great passwordless authentication feature for these linux servers.

**Describe alternatives you've considered**
The other option was to use SSH certificates instead of OTPs but this feature was launched really late and by then we have already adopted with OTP feature.

**Explain any additional use-cases**
It would be great if same can be enabled for RDP protocol for domain joined windows servers with OpenLDAP secret engine which gives AD password for that user.

**Additional context**
We are leveraging Boundary Enterprise (self-managed) already