incomplete API Reference

[The API reference for `ObjectStoreSpec`](https://cloudnative-pg.io/plugin-barman-cloud/docs/plugin-barman-cloud.v1/#objectstorespec) as of v0.9.0 describes the rather important `configuration` field, but instead of referencing a clear description for that field references to the matching [some Go struct](https://pkg.go.dev/github.com/cloudnative-pg/barman-cloud/pkg/api#BarmanObjectStoreConfiguration), which is much harder to read - especially for people not familiar with Go.

Since this is a required part of the configuration, I guess it would be very helpful to have a clear description for this field, like the rest of the API reference.

Start with the ObjectStoreSpec API reference for v0.9.0 and compare its `configuration` field with the linked Go struct, BarmanObjectStoreConfiguration. Find the documentation source for that reference and add a clear description of the required field; done means the generated API reference explains the field without requiring readers to inspect Go code.

Written by the indexing model from the issue text.