refactor: Move base_channel_type_impl to detail namespace

[marco-langer]

Description

This PR moves base_channel_type_impl to detail namespace.

Tasklist

• Ensure all CI builds pass
• Review and approve

[mloskot]

@marco-langer Why closing?

[marco-langer]

@mloskot I wanted to discuss this PR with you before working on it again, but I thought currently the Boost 1.80 PRs are more important.

I think those structs belong to the detail namespace, but they are currently not. However, just moving stuff around would violate boosts deprecation policy, right?

So, some of my solutions:

• Keep the PR as is and have a possible breaking change if someone relied on those structs (I doubt it, but I am not sure).
• Create new detail structs, redirect the library calls to the detail structs and deprecate the old structs in the gil namespace.
• Leave everything as it is currently implemented and don't wake up sleeping dogs.

[mloskot]

OK. You can always turn PR to Draft.

Boost 1.80 PRs are more important.

Yes, indeed.

However, just moving stuff around would violate boosts deprecation policy, right?

Generally speaking, yes.
However, I doubt any user directly refers to base_channel_type_impl type. We can not exclude such possibility though.

Let's postpone it.