Gloas ptc duties beacon node response

[shane-moore]

Changes

• BN will have to be able to respond to VC requests for ptc duties for current or next epoch, per beacon api spec's POST /eth/v1/validator/duties/ptc/{epoch}
• This PR uses vanilla get_ptc to calculate ptc members for a given slot, which is heavy computation when calculating ptc duties for all validators for an entire epoch. But this should be fine for devnet-0
• Future optimizations will be to add in my ptc_cache PR, which allows us cache ptc committees for current/previous/next epoch. Additionally, we'll want out to build out a cache similar to shuffling_cache that allows us to cache instances of the ptc_cache depending upon the current fork choice head (shuffling Id) so that we don't have to recompute ptc_cache when fork choice switches heads and then back again, for example

TODO

• this PR should be repointed at the gloas-validator branch of the conga line once created