Implement AWS SigV4 Header Authorization

[jonatanklosko]

Adds the signing algorithm described here.

The implementation is based on aws-erlang/src/aws_request.erl - sign_request/10 and aws-elixir/lib/aws/signature.ex - sign_v4/5, with no dependencies and some fixes.

Primary differences from the aforementioned functions:

• the function doesn't add X-Amz-Security-Token to the headers, if a security token is to be used, it must be added to the list of headers before invoking the signing function (it's just a regular optional header, not relevant to the signing process)

• when requesting a path like /path%20to the canonical path is computed by URI-encoding the segments, in this case /path%2520to. This behaviour is required for all services except for S3, for S3 the canonical path is just /path%20to. Currently, both packages double-escape URIs for all services, so requests for S3 keys like special&name+noice fail. To address this, the signing function accepts a uri_encode_path: true | false option that controls whether the path gets URI-encoded and it should be set to false for S3. There is a similar :uri_escape_path option in the official Ruby package

[jadeallenx]

sorry late to the party here - is the intention to make this library a dependency of the other packages? Or I suppose maybe there is a desire for a standalone signature library without all of the other code generator-y bits...

[jadeallenx]

Also if the other libraries are wrong shouldn't we fix them too? Or maybe they're getting ripped/replaced? It isn't clear anyway. Thanks.

[jonatanklosko]

@mrallen1 the idea is to use this package as a dependency for the generated packages, so that there is no duplication :)

[jadeallenx]

Super - this is fantastic btw <3 thank you