[Fleet] Make proxy certificate key a secret

[criamico]

Currently the proxy certificate "key" field is saved as a plain text field. We should harmonize it with the other settings and store it as a secret instead:

• Update the api to use secrets. This applies to cases when fleet server is on a minimum version
• For those cases when fleet server is not on this version or secrets are not supported for other reasons, use a encrypted mapping:
  • Register the proxy mapping withencryptedSavedObjects
• Update the UI to have the same appearance currently used in outputs/fleet server hosts/agent binary settings
• Make sure that the generated agent policy contains the updated value
• Make sure that fleet server can read the secret value - see Support new SSL secrets fields fleet-server#4470

As a reference, #213211 can be used.

Current UI:

UI using secrets fields:

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)