Add `base_path` / `path_prefix` configuration option

[pierrelegall]

Problem

Some S3-compatible servers require a path prefix on all requests. For example:

• A MinIO instance behind a reverse proxy at https://example.com/minio/
• An S3-compatible server mounted at a sub-path in a web application, like PS3 mounted via forward "/s3", PS3.Router in a Phoenix router, making the S3 endpoint http://localhost:4000/s3/

Currently there's no way to configure such a prefix — the URL is always built as scheme://host:port/<operation_path>.

Proposed solution

Add a base_path config key that gets prepended to the operation path when building the request URL.

Examples

Service-level configuration:

# PS3 mounted at /s3 in a Phoenix app
config :ex_aws, :s3,
  scheme: "http://",
  host: "localhost",
  port: 4000,
  base_path: "/s3"

# MinIO behind a reverse proxy
config :ex_aws, :s3,
  scheme: "https://",
  host: "example.com",
  port: 443,
  base_path: "/minio"

Per-request override:

"bucket_name"
|> ExAws.S3.list_objects()
|> ExAws.request(base_path: "/s3")