Support labels in "add application" magic link

[r0mant]

Users would like to be able to provide labels in the application install link:

sudo bash -c "$(curl -fsSL 'https://example.com/scripts/<key>/install-app.sh?name=test&uri=https%3A%2F%2Flocalhost%3A443')"

The install script itself supports labels but they can't be passed via this API.

[r0mant]

@ryanclark @marcoandredinis Just FYI. We can probably include this when we're updating Teleport Discover flow for applications (maybe in Q2?)

[marcoandredinis]

We can also re-use the SuggestedLabels prop of the Token.

It would work like this:

• user enters the labels they want (we must limit labels count and label size)
• a new token is created which includes the SuggestedLabels
• when the script is generated, it includes those labels as part of the teleport.yaml config

It currently works for Nodes and only a server side generated Label is present (teleport.dev/resource-id).
However, I think this is something we should converge.

[kimlisa]

this is different from the requested feature from this issue, but there is now support in the web UI for setting labels during the add app discover flows: (see #46976)

https://github.com/gravitational/teleport.e/pull/5810 #50853 #50976