Implement HeaderByHeight in filedao

### What would you like to be added
`HeaderByHeight`, which to be implemented in filedao_v2, should be used instead of `GetBlockByHeight` in https://github.com/iotexproject/iotex-core/blob/1791f82354c672631690e7b759de6bb51cb2a031/blockchain/filedao/filedao.go#L235

```Go
// Before
blk, err := v2.GetBlockByHeight(height)
// After
header, err := v2.HeaderByHeight(height)
```
### Why is this needed
Some queries can be optmized by HeaderByHeight
- https://github.com/iotexproject/iotex-core/blob/1791f82354c672631690e7b759de6bb51cb2a031/api/web3server.go#L850
- https://github.com/iotexproject/iotex-core/blob/1791f82354c672631690e7b759de6bb51cb2a031/chainservice/builder.go#L672

### How important you think this is for IoTeX
- [] must have
- [] should have
- [] nice to have

### Additional information
Let us know any background or context that would help us better understand the request (for example the particular use-case that prompted this request)

Start by reading filedao/filedao.go at the referenced GetBlockByHeight call, then inspect the related query paths in api/web3server.go and chainservice/builder.go. Trace how filedao_v2 retrieves blocks and define the HeaderByHeight behavior needed by those callers. Done means the header-based lookup is implemented and the relevant block lookups use it without changing their results.

Written by the indexing model from the issue text.