Add signer trait

[tdikland]

Add an abstraction for creating signed URLs for the datafiles in (cloud) object storage.

[tdikland]

@roeap

A couple of considerations:

1. I've kept the signature of the signer as simple as possible. We could always add some other methods with default implementation later.
2. For now the signer trait is designed in such that it lives for the duration of the server (for simplicity). A major alternative is to have to instantiate a new signer for every request that fetches file actions from storage.
3. Due to (2) it is a little bit more cumbersome to implement this trait for structs wrapping object_store::signer::Signer (but -of course- it still can be done).

Let me know if you think this is reasonable.

[roeap]

should this take a Url? - not entirely sure, but since we should never be dealing with relative paths o.a. and kernel exposes urls as well i think it may be easier?

[tdikland]

I was not aware that kernel would only exposed parsed URLs. In that case it would make sense to take a url::Url. A (future) consideration could be to take a impl IntoUrl (ref: https://docs.rs/reqwest/latest/reqwest/trait.IntoUrl.html). I'll make the change to url::Url for now

[roeap]

do we need a dedicated type for signed urls? If I think about the flow, we would always get urls from the tables and sign them ad-hoc just before returning them to the client. So are there any situations where we would want to check the properties exposed by this type?

[tdikland]

The protocol prescribes the inclusion of the expiration of the signed URL in the table actions. This type contains both + some convenience methods.

Alternatives:

1. return something like Result<(Url, u64), SignerError> which is harder to grasp and extend.
2. keep track of the expiration time outside of the Signer which would be slightly incorrect.