Add way to iterate config properties in ConfigSourceContext.

[radcortez]

No description provided.

[vsevel]

hello @radcortez. I checked vault's code. I only need to iterate over properties names. so we should be fine with your improvement.

[Emily-Jiang]

What issue is this PR trying to fix?

[radcortez]

What issue is this PR trying to fix?

This is a part of our feature to provide a way to construct a ConfigSource with contextual information from other sources. We are not fixing anything in particular. We are just adding a way for implementors to access the current list of properties from other sources. This will allow to filter sources by a prefix.