Boost fetching from CPM is janky

[blozano-tt]

Today, we use a custom cmake module: fetch_boost.cmake which aims to install boost components for consumption within our project.

This script was copied into tt-train, and tt-umd.

This script causes difficulty in packaging, and in pulling boost from the system.

At the time of this writing, the CPM github page provides an example of the correct way to integrate boost components.
https://github.com/cpm-cmake/CPM.cmake/blob/cd28d445aeb4cecd9d10d63a55a52bcbdd8bf17e/examples/boost/CMakeLists.txt#L14-L21

I don't understand why we wrote our custom module.

CPM github page recommends using Boost 1.86 to avoid bugs in Boost CMake files.

The challenge here is that we need to update two repos, both tt-metal and tt-umd and avoid breaking any consumers.

There are currently two PRs in flight to solve this issue:

#17336
tenstorrent/tt-umd#495

Steps for a smooth transition:

1. TT-M pulls in boost::interprocess

• There is no behaviour change here; we're just fetching this piece of boost earlier in the CMake configure phase

2. TT-UMD switches to clean CPM, but guarded by if(NOT TARGET boost_interprocess

• When TT-UMD builds on its own, it will be using the new mechanism
• When TT-UMD builds as a subproject, it'll use the target brought in by the master project

3. Switch TT-M to the new behaviour
4. (optional) TT-UMD can probably drop the conditional at this point

One sideeffect of the new mechanism is that master projects may need to list the superset of all components that the entire build will require. I'm not certain whether CPM will augment previously imported targets to add additionally listed components. I suspect not.