S3 publisher should support session token

[bboure]

Pre-flight checklist

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project uses.
• I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Problem description

Right now, the s3 publisher does not accept a session token as part of the credentials.
This makes it unusable with something like configure-aws-credentials.

Proposed solution

I think the problem relies here as the AWS_SESSION_TOKEN environment variable is not taken into consideration. There is also no sessionToken option.

I think the best way to handle this is:

• if no credentials options are passed at all, generateCredentials should return undefined, instead of returning env variables explicitely. The AWS SDK should pick them up on its own.
• add a sessionToken option and return it here if users need to specify it manually.

  generateCredentials(): Credentials | undefined {
    const accessKeyId = this.config.accessKeyId;
    const secretAccessKey = this.config.secretAccessKey;
    const sessionToken = this.config.sessionToken;

    if (accessKeyId && secretAccessKey) {
      return { accessKeyId, secretAccessKey, sessionToken };
    }

    // use the SDK default
    return undefined;
  }

Alternatives considered

I see no alternative to this.

Additional information

If this sounds like something that could be done, I'd be happy to open a PR.

Thank you