Address existing issues involving paths in S3 and Signature usages

[oleiade]

This Pull Request results from a debugging call with one of k6's users.

Problem

They were experiencing issues relating to:

• Putting objects whose objectKey would contain space characters.
• Signature path argument would not automatically add a leading slash if missing, leading to erroneous URLs
• The new Endpoint type involved in the signature process is exposed by the signature.js bundle but not the overarching aws.js one.

Solution

To address these issues, this PR applies the following changes:

• We set the S3Client signer's uriEscapePath attribute to true, to ensure paths are always double URI encoded in S3 paths.
• The SignatureV4.sign and SignatureV4.presign operations ensure that a leading slash is appended to the target URL (if not present) when generating the resulting URL. That way we avoid potentially generating malformed URLs such as https://s3.amazonaws.commy-bucket/object-key as we've seen happen.
• We expose the Endpoint type from the index.ts entry point, making it available in the aws.js bundle.

Next steps

In order to unblock our user, I'll publish a v0.12.1 once this is merged.