feat: implement validators rpc

[egonspace]

Closes: #33

Description

By #74, we should use the rpc voters instead of validators.
But it would be more convenient for the client to know both the validators and the voters, so I implemented the rpc validators.

And in #74, lite2's rpc validators has not been modified so I added voters rpc to lite2.

== How to use ==

<start server>
$ tendermint node --proxy_app=kvstore

<start lite client>
$ tendermint lite test-chain-M40t5Z -p "http://127.0.0.1:26657" --height 1 --hash 89032E166DB8B9D50FC7178BF1C44FF756D6EBCA07C20296B6986096140FEA60 -w "http://127.0.0.1:26657"

<rpc test>
// to lite client
curl -s "localhost:8888/validators"
curl -s "localhost:8888/voters"

// to server
curl -s "localhost:26657/validators"
curl -s "localhost:26657/voters"

---

For contributor use:

• Wrote tests
• Updated CHANGELOG_PENDING.md
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Updated relevant documentation (docs/) and code comments
• Re-reviewed Files changed in the Github PR explorer

[zemyblue]

LGTM

[torao]

LGTM