Add AWS credentials resolver code to s3tools

[andyhd]

What

• Copy get_credentials and supporting code from https://github.com/ministryofjustice/aws.signature/tree/credentials-resolver into s3tools
• Update functions to no longer call functions defined in above fork of aws.signature
• This allows us to use the base aws.signature library and not have to maintain our own fork

How to review

You can run the following in your analytics platform Rstudio instance

library(devtools)
install_github("moj-analytical-services/s3tools", ref = "no-fork-aws-sig", force = TRUE)
creds <- s3tools::get_credentials()
Sys.getenv("AWS_SESSION_TOKEN")

This should return a valid AWS token. The creds object can be refreshed (a new token retrieved) using creds$refresh() - this should update the token only if it is is about to expire (or has already).