Refactor GrafanaDashboard to a pydantic base model

[sed-i]

Pydantic 2+ does not allow arbitrary types, so when this lib is consumed, we get a runtime error:

pydantic.errors.PydanticSchemaGenerationError: Unable to generate pydantic-core schema for <class 'cosl.grafana_dashboard.GrafanaDashboard'>. Set `arbitrary_types_allowed=True` in the model_config to ignore this error or implement `__get_pydantic_core_schema__` on your type to fully support it.

cos-lib/src/cosl/grafana_dashboard.py

Line 15 in b15551a

class GrafanaDashboard(str):

A workaround is to add

    class Config:
        arbitrary_types_allowed = True

to pydantic models that rely on it, but that is not a great solution. Instead we could refactor here into a pydantic base model.

Related: canonical/cos-proxy-operator#131