Custom application config, retrievable via a standardized function

## Enhancement Request

### Use Case:
Issue #900 sought to clarify whether `customConfig` was still relevant in the appD spec since the `hostManifests` were introduced. At SWG meeting #1005 a number of participants spoke up for a use case for custom configuration that could be retrieved by an application using a standardized function, where the config might affect the behaviour of the app (and do so in vendor-agnostic fashion). E.g. a market sector visualization or blotter might be filtered by region (APAC, EMEA, etc.).

This is a different use case (intended for apps, vendor agnostic format) to that of the `hostManifests` entry (intended for the DA itself, proprietary format, may be retrievable via proprietary APIs).

The `customConfig` field is currently only retrievable via proprietary APIs as there is no standardized function for retrieving it. Were it to be retrievable in a standardized fashion, it might be used to vary the definition of applications via config alone, in a way that should work in all compliant desktop agents.

Further, it was suggested that the name of the element could also be refined to better identify its purpose, e.g. `applicationConfig`.

Hence, this issue seeks to:
- deprecate `customConfig` in the appD record,
- create `applicationConfig` as an unstandardized object with string field names in the appD record,
- create a means of retrieving any applicationConfig defined for an app in their appD record,
- For example it might be added to the [`ImplementationMetadata`](https://fdc3.finos.org/docs/api/ref/Metadata#implementationmetadata) object returned by `fdc3.getInfo()` as `appConfig` or a dedicated function might be added to the API, e.g. `const config = await fdc3.getAppConfig()`

Start with the appD record definition and the ImplementationMetadata object in the linked API reference, then review how customConfig is currently represented and retrieved. Done means agreeing on the applicationConfig shape, the deprecation path for customConfig, and a standardized retrieval mechanism such as fdc3.getInfo() or a dedicated API function.

Written by the indexing model from the issue text.