[CT-3092] Support optional `label` property for semantic layer objects

[QMalcolm]

Context

We currently allow for specifying an optional label property on metric nodes. This wasn't defined in dbt-semantic-interfaces previously because it was not identified as need in MetricFlow. MetricFlow would like to begin using this property, and would like it to exist on more objects. Thus labels are being added to a number of DSI protocols as defined in dbt-labs/dbt-semantic-interfaces#143. To begin parsing and propagating this labels on Metric nodes, SemanticModel nodes, Dimensions, and Measures.

Acceptance Criteria

• A label can be specified on Metric nodes, SemanticModel nodes, Dimensions, and Measures
• labels on Metric nodes, SemanticModel nodes, Dimensions, and Measures are serialized into the semantic_manifest.json

Blocked By

dbt-labs/dbt-semantic-interfaces#143.

[QMalcolm]

This "technically" isn't blocked. We could add the labels to the noted classes. However, we won't be able to serialize them to the semantic_manifest.json until the the DSI releases the changes.

[emmyoop]

@QMalcolm looking at the blocking issue, is it accurate that Entity should also support an optional label?