[MNG-8576] Investigate modifying the Toolchain API to be session/project scoped

**[Guillaume Nodet](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=gnt)** opened **[MNG-8576](https://issues.apache.org/jira/browse/MNG-8576?redirect=false)** and commented

We do underuse the scopes as we tend to pass the `Session` in all requests to have stateless services.

The `ToolchainManager` is a good example, as we do actually get the mojo execution context to get to the project and store/retrieve the selected `Toolchain` according to that.

That looks wrong and should be simplified.

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**Affects:** 4.0.0-rc-3

**Issue Links:**
- [MNG-8558](https://issues.apache.org/jira/browse/MNG-8558) Switch DefaultToolchainManager to Api V4