[Change Proposal] Add a new discovery parameter that allows any key / value pair to be set

Currently, content packages are allowed to define different fields to allow for discovery and since https://github.com/elastic/package-spec/issues/919 it also allows datasets names to be set.

The goal of this issue is to add another parameter to allow discovering packages based on any key / value pairs that could be provided.

Example of how this new parameter could be defined in the manifest:

```yaml
discovery:
fields:
- name: host.ip
value: xx
datasets:
- name: nginx.access
```

## Use Cases

### RUM

RUM data is coming from OTel SDKs, hence all the data lands in `traces-generic.otel-default`.
To allow auto-install of a RUM content pack, we would need the possibility to check for the `telemetry.sdk.name` field to contain the value `webjs`.

### Data from generic OTel Collector Receivers

Data from generic receivers (e.g. `prometheusreceiver`) would land in a dataset like `prometheusreceiver.otel`. However, that data may contain metrics from different kind of technologies. To auto-install a content pack for a specific technology in the prometheus metrics, we might need something similar as the above as well.

### Requirements
- [ ] Add new discovery parameter `discovery.fields.value`.
- [ ] Value field is optional, if not provided it will check for existence of the field, otherwise check for the specific value