meta request: support for should_continue cancellation callback

[grrtrr]

This restores an updated version of #238.

Rationale

Add a boolean continue_callback, which polls a request-cancellation flag set by the user. This matches the use-case of the ContinueRequestHandler / ContinueRequest / ShouldContinue boolean functions used by aws-sdk-cpp.

Passive polling is preferred over active cancellation, since the latter would require to store a reference to the s3 meta request. The polling function is evaluated at the following points during the lifetime of the meta request:

1. Request preparation (aws_s3_meta_request_prepare_request).
2. Request update (the .update handler invoked via aws_s3_meta_request_update).
3. Http stream completion (.on_complete handler invoking aws_s3_meta_request_send_request_finish_default).
4. Writing download data (aws_s3_meta_request_stream_response_body_synced).

If a continue_callback is provided and if it returns false at any of these 4 evaluation times, the meta request completes with the error_code AWS_ERROR_S3_CANCELED.

By using a boolean flag in the body of the function, the polling overhead can be kept minimal.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[grrtrr]

@jmklix @TingDaoK - this is an updated version of #238, which takes into account Dengke's changes in #395 and #399.

We need the should_continue callback function in order to implement the AWS C++ SDK Cancel() function for the S3CrtClient.