feat: implement get_filter_changes

[sk1122]

Ref - #200

Implemented eth_getFilterChanges according to discussion

Do not merge this yet

• [ ] eth_getFilterChanges works for eth_newFilter, eth_newBlockFilter which returns different types of data (Vec<Log> & H256)

[sk1122]

Hey @ncitron, sorry for not updating, I am facing an issue in rust, get_filter_changes returns Vec<R>, R is a generic type, it can either be Vec<Log> or Vec<H256>, tried converting the result of ethers function to any and then to an Enum of both the types, but receiving another error

type inside `async` block must be known in this context
cannot infer type for type parameter `R` declared on the associated function `get_filter_changes`

can you help in this?

[ncitron]

Can you push what you have and point me to where you are getting an error?