[FEA] Consider adding an instantiation of raft::linkage::connect_components

[ahendriksen]

Is your feature request related to a problem? Please describe.

The raft::linkage::connect_components uses fused_l2_nn with a custom operation. This is not instantiated in libraft.so. This was brought up in PR #1439 (5171de3#r1173625212)

Describe the solution you'd like
Options:

• Add an instantiation for raft::linkage::connect_components in libraft.so
• Instantiate fused_l2_nn in the test for raft::linkage::connect_components in a separate source file (to make the tests compile fast)

Describe alternatives you've considered

Additional context
Discussion:

• 5171de3#r1173625212
• 361570b#r1173629689