Add customizable Progress

[fujidaiti]

• [x] I’ve reviewed the contributor guide and applied the relevant portions to this PR.

This PR adds a customizable Progress mentioned in #85.

Demo:

https://github.com/dart-lang/cli_util/assets/68946713/3d719207-dc18-466f-bbb8-762a0124cd90

Changes made:

• Add CustomProgress class that can display an arbitrary single line message with a specific time period
• Add an example of 'CustomProgress'

Consideration:

• How should this new progress be integrated with Logger.progress()?

I hope you like these changes, thanks!