Switch to OpenDAL's S3 credential loader

[dfunkt]

One of the reasons to use a custom credential loader was, for example, OpenDAL's lack of AWS SSO temporary credentials.

vaultwarden/src/config.rs

Lines 1399 to 1402 in b04ed75

	// This is a custom AWS credential loader that uses the official AWS Rust
	// SDK config crate to load credentials. This ensures maximum compatibility
	// with AWS credential configurations. For example, OpenDAL doesn't support
	// AWS SSO temporary credentials yet.

Now that support for this type of credential has been added (https://docs.rs/opendal/latest/opendal/services/struct.S3.html#temporary-security-credentials), perhaps it can be used. It would allow to drop a fair bit of dependencies that increase the build surface.