[kernel 1/n] Introduce initial libbitcoinkernel

Summary:
This diff introduces a libbitcoinkernel library linking in the minimal list of files necessary to use our consensus engine as-is. bitcoin-chainstate introduced in D15015 now will link against libbitcoinkernel.

Note that for now there is no good reason to introduce the new BUILD_BITCOIN_KERNEL_LIB option, we can gate both the bitcoin-chainstate executable and the bitcoinkernel lib with the same BUILD_BITCOIN_CHAINSTATE option until it makes sense to split them.

This is a backport of core#24322

Depends on D15015

Test Plan:

cmake .. -GNinja -DBUILD_BITCOIN_CHAINSTATE=ON
ninja

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15028