[Request] - support encrypt files before pushing to object store

As best as I can tell, the only [options for tuning encryption](https://pkg.go.dev/github.com/cloudnative-pg/barman-cloud/pkg/api#EncryptionType) are 'passed through' to the underlying `s3` (or compatible) API calls.

If it turns out that I'm wrong on that one, please let me know!

My question/request: allow use of symmetric encryption _before_ the files leave the host and are sent off to the object store.

I think this could be as straight forward as passing in a key/cypher to the [underlying archiver](https://github.com/cloudnative-pg/plugin-barman-cloud/blob/9be87b0d9c87c0df7f1f72caa33bf848fbc16582/internal/cnpgi/common/wal.go#L147)

The motivation: not all object stores support "BYO" key for bucket encryption and even if they do, you're still required to hand the sensitive key material off to the remote backend so that it can encrypt things for you. One of the simplest ways to address this is to encrypt objects before they're ever off the host.
Symmetric encryption is sufficient for "at rest, I need to know that I truly am the only person that can read it" needs.

Start with the EncryptionType API and internal/cnpgi/common/wal.go at the linked archiver call. Trace how WAL files are handed to the archiver and how encryption options reach the object-store API; done requires a defined symmetric-encryption configuration and verified protection before upload.

Written by the indexing model from the issue text.