feat: replace foundation/token by token-source

[nicolas-vivot]

@yoshidan

As discussed here, i have created the token-source crate on this repository.

• I have invited you on the repository as contributor/maintainer.
• Let me know if you also want me to add you as maintainer on crate.io for this crate.

Code has been migrated 1 to 1.
I just added a feature called async-token-source to provide both sync and async TokenSource trait, to avoid to enforce any async for people who do not need an async source.

I have replaced the use of the internal foundation/token by token-source everywhere. Please double check in case i missed a location, i did a quick search & replace.

[yoshidan]

Thank you for your contribution.