[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.

EncryptionType API と internal/cnpgi/common/wal.go から、リンクされた archiver 呼び出しを起点にしてください。WAL ファイルがどのように archiver に渡され、暗号化オプションがどのように object-store API に到達するかを追跡してください。完了には、定義済みの対称暗号化設定と、アップロード前に保護されていることの検証が必要です。

索引モデルが issue の本文から書いたものです。