Entity attributes management framework.

## Pluggable Entity Attributes package

The package supports entity attributes management. Attributes can be provided by plugged in providers, The implementation caches providers results.

### Clients Interface

Clients should start at {@link Turbo} class. It allows to query for attribute values, to determine their readiness, to write them down and supports listening on attribute changes.

### Providers Interface

Attribute providers should start at {@link FileAttributeProvider} implement it and register it using system filesystem at `Services/FileAttributeProviders/Default`.

### Consistency Note

It may happen that value changes at providers level, however unnoticed by clients because the value is cached at memory layer and framework does not check providers in such case.

It may happen only if provider does not have exclusive control over stored data (e.g. CVS/Entries based provider and extenally executed cvs).

Solution is that this risk is documented at attribute definition contract and client of such contract provides to user explicit action that synchronizes states. Contract should define `null` value for invalidation purposes.

### Attribute Definition

Both providers and clients must understand attribute definiton they are using. The attribute definitions are not covered by this package.