Add support for encrypted credentials to promote self-service

[Racer159]

Is your feature request related to a problem? Please describe.

As Key I want to be able to access a web portal to encrypt renovate secrets just like I do on the internet-facing mend site.

Describe the solution you'd like

• Given I have a secret that I want to place in a renovate config
• When I navigate to renovate.<uds.domain>
• Then I am presented with the Mend encryption page
• And When I enter a secret and receive a new one back
• And Add it to my renovate config
• Then Renovate will pickup and use the secret appropriately

Describe alternatives you've considered

We could include secrets within k8s / the renovate app itself but this flow would require a user to go through their environment's admin to get the secrets added - this may encourage them to just commit the plain text secrets to the repo (since this would also work and is faster for their workflow)

Additional context

Even though this is a public key - ideally this would be hosted behind authservice to provide an extra level of protection.