Add optional inputId property to InputSpec interface

[chrispcampbell]

To support the improvements being made to the model-check tool in #315, it would be helpful if the InputSpec interface (from the build package) included an optional inputId property.

When the inputId is defined, it provides a more resilient way to compare a scenario between two versions of the model even if the variable is renamed.

The plugin-config package already includes an id column in inputs.csv for this purpose, so we just need to include that when initializing the InputSpec for each row.

Proposed API:

export interface InputSpec {
  /**
   * The stable input identifier.  It is recommended to set this to a value (for example, a
   * simple number like what `plugin-config` uses) that is separate from `varName` and is
   * stable between two versions of the model.  This way, if an input variable is renamed
   * between two versions of the model, comparisons can still be performed between the two.
   * If a distinct `inputId` is not available, plugins can infer one from `varName`, but
   * note that this approach will be less resilient to renames.
   */
  inputId?: string
  // ...
}