Creating a client secret

[NvanWeeghel]

Userstory:
As a developer, I want to be able to create a new client secret so I can replace the expired secrets with secrets that still work.

Acceptance criteria:

• The developer must be authorized to be able to create a secret.
• It must be possible to enter a description.
• It must be possible to enter an expiration date.
• The secret must be created by the application.
• The secret must be correctly associated with the client.
• The secret must be visible in the application.
• The created secret must be shown to the user once.

[jmezach]

@NvanWeeghel Technically I think there can be multiple active (ie. non-expired) secrets for the same client so this story isn't only to replace an expired secret with a new one. The same functionality can also be used to just generate an additional secret for the same client.

[jmezach]

@NvanWeeghel I've also updated the title of this story to reflect that this is scoped to client secrets. IdentityServer also has the notion of API Resources which can also have associated secrets, but as far as I'm concerned this story only relates to client secrets. Perhaps we could reuse some of this for API Resource secrets as well, but we'll cross that bridge when we get there.