bug: fork choice does not count block proposer's attestation

[unnawut]

I'm working on this but adding this issue for tracking because it's taking longer

Currently to write test for #160 that requires a certain number of attestations to trigger justification, I found that the proposer's attestations are not being counted and so the proposer's attestations have to be added manually: https://github.com/leanEthereum/leanSpec/pull/160/files#diff-c13118c4a040ea99650d2c72b4961cd2483ccbcb84ef9d16eb96751cfc8ec12cR881-R896

I suspect some logic is missing in fork choice store's on_block() or on_attestation() but still looking into it

Once this is done, the 2 proposer attestations workaround in #160 (comment) should be removed

[KolbyML]

I don't think this is a bug, because the validator builds the block and adds it's attestation to the end of the block. The block builder doesn't need to propgate its attestation over p2p, hence it wouldn't be addable from the fork choice logic