Vault UI Support for Custom Secrets Engines

[SamuZad]

Is your feature request related to a problem? Please describe.
I am working on a custom secrets engine that will be able to issue "keys", mostly for non-technical users. The plugin is feature-complete, however, we can't seem to get it to "work" through the UI

It seems like the Vault UI does not support interacting with custom secrets engines. At least, it would seem so, since none of the secret engines I download (or created) seem "intractable" via the UI.

The last relevant information I can find around this is a comment over 4 years old: https://discuss.hashicorp.com/t/not-able-to-access-secret-plugin-created-in-the-ui/6852

Describe the solution you'd like
It would be amazing if the UI supported custom secrets engines, like it does databases. Or if there is a way for a plugin to "signal" for UI availability that we couldn't find, that would be amazing as well

To be clear, I'd be happy to contribute to this effort - but maybe there is a good reason this isn't supported that I'm not aware of?

Describe alternatives you've considered
We have considered "masking" our plugin to appear to be a database plugin, but ultimately decided against it - as it probably wouldn't be great in the long run

Explain any additional use-cases
Programmatic/CLI access is fine for technical users, but when it comes to driving Vault adoption for non-technical users, it would be great if custom secrets engines were better handled

Additional context
Inspired by the amazing work on #27413