[CT-3246] Make specifying semantic layer object `label` attributes optional

### Housekeeping

- [X] I am a maintainer of dbt-core

### Short description

Currently different semantic layer objects treat their `label` attributes differently. Additionally on some semantic model objects `label` is `Optional`, meaning that consumers of the manifest have too look at both the `name` and `label` of a semantic layer object to determine what its "Display Name" should be.

To simplify things for people writing yaml, we want to make specifying a `label` optional. Additionally we want to make life easier people consuming the manifest to only have to look in one place, `label`, for a semantic layer object's "Display Name".

### Acceptance criteria

For the following objects `metric`, `semantic_model`, `saved_query`, `dimension`, `entity`, or `measure`
* When defining in YAML, the `label` attribute should be optional
* If the `label` attribute is undefined in YAML, it should be defaulted the object's `name` value

### Impact to Other Teams

The semantic layer will be able to depend on the `label` attribute being present

### Will backports be required?

Required? No

### Context

_No response_