Allow custom sendFile() function for awsS3

[ajmas]

Based on the work on issue #276 I felt, at least for the scenario of my project, that it would be useful to have a custom sendFile() process, not limited to S3. For this reason maybe we could include a 'sendFile' option which would specify a function that returns a promise?

[ajmas]

A first version is compromise to allow the awsEndpoint.sendFile to be replace with a user specified version, but ideally this could be larger than just S3:

 getSignedAndUploadToS3(file) {
      let sendFile = awsEndpoint.sendFile;
      
      if (this.awss3.sendFile && typeof this.awss3.sendFile === 'function') {
          sendFile = this.awss3.sendFile;
      }

      sendFile(file, this.awss3)
        .then((response) => {
          if (response.success) {
            file.s3ObjectLocation = response.message
            setTimeout(() => this.dropzone.processFile(file))
            this.$emit('vdropzone-s3-upload-success', response.message);
          } else {
            if ('undefined' !== typeof message) {
              this.$emit('vdropzone-s3-upload-error', response.message);
            } else {
              this.$emit('vdropzone-s3-upload-error', "Network Error : Could not send request to AWS. (Maybe CORS error)");
            }
          }
        })
        .catch((error) => {
          alert(error);
        });
    },

Update: based on further testing if we do actually upload via sendFile, then dropzone still tries to upload at the URL specified via the 'url' parameter, which is not ideal.