Credential refactor #2972

This is derived from a thread in #2836 (comment) and from @plusplusjiajia's original implementation on #2838. There's three issues I see with the freshly introduced iceberg::catalog::session::Credential type:

1. the naming of Credential already exists in the context of credential vending (see iceberg_catalog_rest::StorageCredential - but in Java it's plain Credential). A new Credential concept meaning "any sensitive string" conflicts with it
2. use cases exist for a sensitive string type outside of catalog credentials, like FileIO properties (e.g. s3.session-token): iceberg::catalog is the wrong place to share it
3. The existing (and analogous) iceberg::encryption::SensitiveBytes type is located somewhere completely different, making it non-obvious that both exists and share semantics