CoRIM store migration

[setrofim]

Switch to using CoRIM store, rather than the key-value store, for endorsements and trust anchors. Endorsement and trust anchors are now stored in a format that preserves the entire CoRIM/CoMID contents, and is entirely scheme-agnostic. The more sophisticated structures allows for more sophisticated a management life cycle to be implemented in the future, and the more generic structure makes scheme implementation easier.

This change has a huge knock-on effect on both provisioning and verification pipelines implementations, and the scheme implementation framework has been completely restructured:

• IEvidenceHander, IEndrosementHander, and IStoreHander interfaces are gone, replaced by the single ISchemeHander interface.
• A generic scheme wrapper implements ISchemeHander based on a smaller ISchemeImplementation interface and a SchemeDescriptor. Most schemes just need to implment ISchemeImplementation and provide the SchemeDescriptor.
• The SchemeDescriptor is a declarative element, grouping static information about the scheme, such as its name, supported media types, etc (in the past, this was defined on ad-hoc basis using multiple variables within old scheme implementations).
• Scheme version has also been added as part of the descriptor. This isn't really used at the moment. All existing schemes have been set to version 1.0. (note: this isn't used at the moment)
• ISchemeHander/ISchemeImplementation expose similar API to the old IEvidenceHander (verification pipeline stages remain the same), but the argument types differ; e.g. "keys" for endorsement/trust anchor lookup are now represented as comid.Environments.
• Instead of IEndrosementHander methods, ISchemeHander has a single ValidateCorim (made option in the ISchemeImplementation by the wrapper). This allows schemes to supply custom validation for CoRIMs before they are added to the store (this can also be more cleanly done via CoRIM profiles).
• IStoreHander mothods are no longer necessary as the store interface is entirely scheme-agnostic.
• Protobuf types have been removed from the IStoreHander methods. Protobuf is an implementation of the underlying RPC mechanisms and should not be exposed to the schemes (effectively burdening them with partial serialisation for transport).

CoRIM store is initialized and managed by its own client. Deployments have been updated to integrate it.

Since there is now a single scheme interface, the combined/split plugins configuration has been removed. There is now always one plugin per scheme. Plugin executables have also been renamed to be prefixed with scheme- or coserv- depending on plugin types (as CoSERV plugins currently reside under scheme/).

Since there is now a single interface for the entire scheme, IPluggable.GetSupportedMediaTypes() has been changed to return a map[string][]string instead of just []string. This allows to separate provisioning from verification media types.

An example "scheme" has been added containing the boilerplate template for implementing new schemes.

[thomas-fossati]

A regression I noticed en passant.