Allow for specific bundle paths

[jardon]

Enhancement Proposal

Currently, the juju-bundle implementation in the upload-bundle action changes directories to the specified path and looks for a bundle.yaml file as seen in here:

import * as core from '@actions/core';
import * as exec from '@actions/exec';

class Bundle {
  async publish(path: string, channel: string) {
    core.exportVariable('CHARMCRAFT_AUTH', core.getInput('credentials'));
    process.chdir(path);
    await exec.exec('juju-bundle', [
      'publish',
      '--destructive-mode',
      '--serial',
      '--release',
      channel,
    ]);
  }
}

export { Bundle };

This unnecessarily enforces bundle names which creates unnecessary rigidity to how bundle repos have to be structured. Not only do the bundles have to be named bundle.yaml, but they also will have to be in separate directories. This makes little sense when a path to a specific bundle can be given straight to juju-bundle using --bundle.