Add organizational secrets API

[pigeonhands]

Adds apis for creating and deleting organizational secrets.

let org = client.orgs("owner");
let secrets = org.secrets();

let pub_key = secrets.get_public_key().await?;

secrets.create_or_update_secret("GH_TOKEN", &CreateOrganizationSecret{
     key_id: pub_key.key_id,
    encrypted_value: String::from("some-b64-string"),
    visibility: Visibility::Private,
    selected_repository_ids: None,
}).await;

let secret_info = secrets.get_secret("GH_TOKEN").await?;

secrets.delete_secret("GH_TOKEN").await?

[maflcko]

You may have to run cargo fmt?

[pigeonhands]

You may have to run cargo fmt?

Done

[XAMPPRocky]

Thank you for your PR!