Remove dependency on near-sdk-rs

[matklad]

At the moment, indexer depends on near-sdk-rs:

https://github.com/near/near-indexer-for-explorer/blob/a6dbc3c1f1f1e6a8727b588713e5ee66a6ad0519/Cargo.toml#L27-L28

This doesn't make physical sense: near-sdk-rs is essentially a wasm-only crate for writing smart contracts, it just happens to work on x86_64-unknown-linux-gnu, but this is not guaranteed.

Looking at the usages, it is only pulled for some serialization utils. These utils should be re-implemented here instead. There's very little meaningful code there, so any perceived code sharing benefits are dominated by accidental coupling between unrelated projects.

Context: https://near.zulipchat.com/#narrow/stream/295558-nearinc-protocol-core/topic/newbie.20questions.20about.20localnet

[khorolets]

cc @telezhnaya

[frol]

The idea behind it was to copy-paste the lockup contract as is with minimal modifications, but I agree that might be an overkill to justify pulling the whole near-sdk. It is not urgent or important for us at the moment, so I will mark it as good first issue for whoever will be willing to put energy into slimming the dependencies, learning more about lockup contract and circulating supply.