[Feature request] Support custom variable name in "upload_name" field

When not specified, upload_name uses $CIRCLE_BUILD_NUM, which I found in the changelog. The docs suggest an empty string:

☝ this could be treated as a small "bug" in docs 😉 but I actually wanted to ask for a feature.

It would be nice to name uploaded reports per container. Unfortunately, CircleCI doesn't support environment variable interpolation in yaml so something like this:

upload_name: "container_${CIRCLE_NODE_INDEX}"

is going to result in the name as we see it:

But I'd be totally fine with preparing the variable name in a previous step, like this, and using it later:

- run:
    name: Set CODECOV_UPLOAD_NAME
    command: echo "export CODECOV_UPLOAD_NAME=container_${CIRCLE_NODE_INDEX}" >> $BASH_ENV
- codecov/upload:
    upload_name: "$CODECOV_UPLOAD_NAME"

or to be more explicit:

- upload_name: "$CODECOV_UPLOAD_NAME"
+ upload_name_var: "$CODECOV_UPLOAD_NAME"

With whatever priority upload_name vs upload_name_var should have in case they are used together.

What do you think?