Add support for assuming roles from web identity

[markdouthwaite]

Please specify whether your issue is about:

• a possible bug
• a question about package functionality
• a suggested code or documentation change, improvement to the code, or feature request

Description

It'd be great if this package supported assuming roles from web identity tokens:

Calling AssumeRoleWithWebIdentity does not require the use of AWS security credentials. Therefore, you can distribute an application (for example, on mobile devices) that requests temporary security credentials without including long-term AWS credentials in the application. You also don't need to deploy server-based proxy services that use long-term AWS credentials. Instead, the identity of the caller is validated by using a token from the web identity provider.

[parul157]

Hi @jon-mago, thanks for merging the PR.
Can we also publish a new version and do CRAN release if possible?

[jeffqumata]

Are there any plans to actually release this via CRAN? Theres not been a release there since 2020-06-01 :(