Add package signing plugin example using OpenSSL

[danimtb]

Use the examples with Conan client changes at conan-io/conan#19345

[czoido]

All this code should be explained in an example, in the examples section of the docs: https://docs.conan.io/2/examples.html as a part of this PR: conan-io/docs#4365 or in a separate one if you prefer.

[danimtb]

All this code should be explained in an example, in the examples section of the docs: https://docs.conan.io/2/examples.html as a part of this PR: conan-io/docs#4365 or in a separate one if you prefer.

I agree! Let's add it in a different PR

[czoido]

I'm a bit concerned about not being clear that this is just an example and we are not taking into account best practices for security, may we add a comment in the code similar to this:

"""
SECURITY NOTE:
This example stores a private key next to the plugin for simplicity. Do not do this in production.
Instead, load the signing key from environment variables or a secret manager, or delegate signing to a remote signing service.
Always keep the private key out of the Conan cache and out of source control.
"""

and also notes in the READMEs?