@actions/artifact: Support custom Octokit client

Describe the enhancement
downloadArtifact function does not retry if GitHub API returned an error such as status 500. This is because the Octokit client is hardcoded as below:

https://github.com/actions/toolkit/blob/1b1e81526b802d1d641911393281c2fb45ed5f11/packages/artifact/src/internal/download/download-artifact.ts#L131-L147

It would be nice if downloadArtifact function supports an option to pass an Octokit client. Here is an example snippet:

import * as github from '@actions/github'
import { retry } from '@octokit/plugin-retry'
import { DefaultArtifactClient } from '@actions/artifact'

const example = () => {
  const octokit = github.getOctokit(token, {}, retry)

  const artifactClient = new DefaultArtifactClient()
  artifactClient.downloadArtifact(artifactId, {
    path: 'example-directory',
    findBy: {
      workflowRunId: workflowRunId,
      repositoryOwner: github.context.repo.owner,
      repositoryName: github.context.repo.repo,
      octokit: octokit,
    },
  })
}

Start with packages/artifact/src/internal/download/download-artifact.ts at the linked hardcoded Octokit client and trace the downloadArtifact options and findBy types. Check the existing artifact tests before making changes; done means callers can provide a custom Octokit client for artifact downloads and that the client is used for the GitHub API request.