Create a consistent Connection interface and organize structs into individual files.

[bdhobare]

• In this PR, we are implementing a sol::Connection class inspired by the web3.js sdk and organize the library into a consistent library in solana.h.
• We have also moved the existing structs into separate files where each RPC call will be handled separately to avoid crowding one file.

The implemented API is illustrated in examples/connection-example.cpp. It looks like this:

#include "solana.h"

// Example: getAccountInfo RPC method

const auto rpc_url = "...";
const auto pubKey = "..";
sol::Connection connection = sol::Connection{rpc_url, "finalized"};
auto key  = sol::PublicKey::fromBase58(pubKey);
std::optional<sol::AccountInfo> info = connection.getAccountInfo(key);
if (!info.has_value()){
    std::cout << "Account doesn't exist.\n";
    return;
}
std::cout << "Account Owner: " << info.value().owner.toBase58() << std::endl;

Implementing a new RPC method

Add it to connection.h and implement it in a separate file. Example:

• All RPC calls involving Accounts are in Account.h
• All RPC calls involving transactions are in Transaction.h

The added method will be used by the library user as:

auto response = connection.getXXYY(request);

WebSockets

This PR starts with the HTTP RPC methods(only getAccountInfo is implemented here) to get feedback and websockets implementation will be added in follow up PRs.

Next Steps

• [ ] All the existing examples and methods in solana.hpp by @mschneider have not been modified. We can start looking at migrating them to use the sol::Connection interface.
• [ ] Implement all the HTTP RPC calls in connection.h
• [ ] Add support for websocket to sol::Connection instance.
• [ ] Support templated AccountInfo<T>
• [ ] Support multiple concurrent connection instances.
• [ ] Add more test coverage.
• [ ] Fix warnings/deprecations.