add sts_http_proxy and sts_endpoint_url to web_identity_credentials

[rubroboletus]

possibly solves #451

[daipom]

Could you add DCO?

https://github.com/fluent/fluent-plugin-s3/pull/452/checks?check_run_id=36048644495

[daipom]

@rubroboletus Thanks for this enhancement!!
So, this PR adds the same options to web_identity_credentials as assume_role_credentials.
It looks good to me.

If you can, could you please add a note to the next document about these options?

• https://github.com/fluent/fluent-plugin-s3/blob/master/docs/credentials.md

Since there is no description of these existing options in assume_role_credentials, it would be helpful if you could add a description for both assume_role_credentials and web_identity_credentials.