[Bug] Plugins are always used to archive WAL files if they have the capability

`isWALArchiver` is ignored and [plugins are always used to archive WAL files](https://github.com/cloudnative-pg/cloudnative-pg/blob/3f2626d709c24ffdb3fb325e2fb09861d816467c/pkg/management/postgres/archiver/archiver.go#L298) if they have the capability to do it.

Given the following plugins stanza:

```yaml
plugins:
- name: barman-cloud.cloudnative-pg.io
enabled: true
isWALArchiver: false
parameters:
serverName: cnpg-default
barmanObjectName: cnpg-default-barman
```

You would expect WAL not to be archived using this plugin. However, there is no filtering in the [archiveWALViaPlugins](https://github.com/cloudnative-pg/cloudnative-pg/blob/3f2626d709c24ffdb3fb325e2fb09861d816467c/pkg/management/postgres/archiver/archiver.go#L277). It always archives with plugins that support WAL archiving.

The only cases where it won't archive WAL are:

1. if you specify a plugin as the WAL archiver and it doesn't have that capability.
2. you don't specify any plugins