contracts: Make `Origin` information available

[athei]

As of right now a contract needs to implement a government model for privileged operations. Examples of such operations would be upgrading the code of a contract or self destruction.

It would be nice if a contract could tap into a chains government system when they align. In order to do so we would expose an API to contracts that allows checking if it was called by a special origin instead of an AccoundID.

We would probably mirror the tracks used in referendas (root, whitelisted, spender). This allows contracts to implement something like ensure_root()?.

Was brought up here: https://substrate.stackexchange.com/questions/7608/ensure-root-in-ink

If we should support only root specifically or go with a more generalized API is still an open question. More research needed.

[athei]

Result of a discussion with @juangirini:

• We just support root for now
• Storage deposits are neither charged nor refunded in case of a root origin (this is okay because refunds are pro rata)
• The caller API traps in case the caller is root (as oposed to copying a fallback value)
• We create our own Origin enum that we use throughout our code. It is created by a TryFrom<RuntimeOrigin> impl that checks that only supported origins are used.