util: Introduce `get_key`/`get_keys` in `Keychain` + `KeychainServer`

[xdustinface]

This is preparation for the key name support where in a follow up the key name gets added to KeyData. Adds the two new methods:

• get_key to fetch a KeyData entry based on a fingerprint
• get_keys to fetch the KeyData entries of all keys in the keychain.

There is a parameter include_secrets (defaults to false) in the related methods and the server requests which allows to hide/show the secrets.

Based on:

• #12772
• #12752

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