Add Spanner schema for UMA and Chromium Enums

[jcscottiii]

This schema will enable efficient storage and querying of UMA telemetry data, specifically focusing on Chromium enums and their associated metrics.

Part of splitting up #616

ChromiumHistogramEnums:

• Stores histogram names (e.g., WebDXFeatureObserver) with a generated UUID as the primary key.
• Designed to support multiple histogram types in the future.
• Adds a calculated lowercase column for case-insensitive searching.

ChromiumHistogramEnumValues:

• Stores the values within a given enum.
• Each entry includes a generated UUID, a foreign key referencing ChromiumHistogramEnums, the BucketID, and a human-readable Label.

WebFeatureChromiumHistogramEnumValues:

• Maps web features to ChromiumHistogramEnumValues.
• Specifically designed for the WebDXFeatureObserver enum.

DailyChromiumHistogramMetrics:

• Contains daily metrics for each value in ChromiumHistogramEnumValues.
• Each row represents a feature key, day, and metric value.

DailyChromiumHistogramEnumCapstones:

• Records completed enum ingestions for a particular day.

A future PR will update the database documentation.