Add Spanner functionality to store mappings between Enums & Features

[jcscottiii]

This change builds on the landed schema changes in #665.

This change adds functionality to store mappings between web features (Table: WebFeatures) and Enum Values (Table: ChromiumHistogramEnumValues)

Since the metrics are given by enum value ID, this mapping will be useful for looking up the metrics for a given features

Table: WebFeatureChromiumHistogramEnumValues

• Implements the writeableEntityMapper interface
• Exposes one method:
  • UpsertWebFeatureChromiumHistogramEnumValue - used to insert the mappings

Part of splitting up #616