declare that fs.s3a.ext. is a prefix for arbitrary extensions

it turns out to be useful to use per-bucket config extensions for things like delegation tokens.
To avoid third-party extensions to pollute the fs.s3a. namespace with their own config options (So meaning its inevitable things break when we add some new fs option), I propose the following policy

1. applications/extensions MUST NOT add their own config options under fs.s3a.
2. anyone is free to use "fs.s3a.ext.$product." options in s3a.
3. we promise not to go near those options in the S3A module
4. it is left to people who add their own settings here to avoid clashing with others, This is why you should use a unique product name

With this specified (where?) then if someone uses some fs.s3a. option which gets broken by an updated s3a connector, well, we say "wontfix", —and indeed, get to tell them off for doing this.

Propose: similar for the other stores