Initial version with PKAM auth and simple data exchange

[Zambrella]

Description

This is an initial version of a Rust Client SDK working in std environments. This version of the SDK can do everything described here which includes:

1. Lookup the address of an AtServer from the AtDirectory
2. PKAM authenticate with AtServer
3. Read .atKeys file and decrypt data contained within
4. Send encrypted data to another AtSign
5. Read encrypted data sent from another AtSign

How to verify

There are instructions on how to run the examples in the README. Alternatively, you can add it as a dependency in a new Rust project.

[dependencies]
at_rust = { git = "https://github.com/atsign-foundation/at_rust.git" }

[cpswan]

Tremendous work @Zambrella - I was able to get the examples working without too much trouble.

A few nits here and there to be tidied up before merging, but mostly LGTM :)

[Zambrella]

@cpswan I have pushed changes that hopefully resolve all the issues.

[cpswan]

LGTM @Zambrella over to you for the merge