[r2r] Allow passing Zcash params file path to activation request

[laruh]

related to ARRR integration. #927

Now we can pass zcash_params_path parameter to Zcoin activation request.

curl example with path param:

"{\"userpass\":\"${USERPASS}\",\"method\":\"task::enable_z_coin::init\",\"mmrpc\":\"2.0\",\"params\":{\"ticker\":\"ZOMBIE\",\"activation_params\":{\"mode\":{\"rpc\":\"Light\",\"rpc_data\":{\"electrum_servers\":[{\"url\":\"zombie.sirseven.me:10033\"}],\"light_wallet_d_servers\":[\"http://zombie.sirseven.me:443\",\"http://zombie.sirseven.me:1443\",\"http://zombie.sirseven.me:2443\"]}},\"zcash_params_path\":\"/home/user/TEST_PATH/.zcash-params\"}}}"

The tree example for easy reading looks like:

{
  "userpass": "${USERPASS}",
  "method": "task::enable_z_coin::init",
  "mmrpc": "2.0",
  "params": {
    "ticker": "ZOMBIE",
    "activation_params": {
      "mode": {
        "rpc": "Light",
        "rpc_data": {
          "electrum_servers": [
            {
              "url": "zombie.sirseven.me:10033"
            }
          ],
          "light_wallet_d_servers": [
            "http://zombie.sirseven.me:443",
            "http://zombie.sirseven.me:1443",
            "http://zombie.sirseven.me:2443"
          ]
        }
      },
      "zcash_params_path": "/home/user/TEST_PATH/.zcash-params"
    }
  }
}

Without the path parameter the default path will be used, which is home directory plus the name of zcash params folder in current OS. Like it was before this PR.

[artemii235]

Thanks for the PR! One note.

[artemii235]

As there are FS operations and LocalTxProver::new can take significant time to load, please wrap this entire code block into async_blocking.

[laruh]

Oh, I see.
Done

[artemii235]

Great! @laruh Could you please provide examples for docs updates to @smk762?

[smk762]

Thanks :)
Draft zhtlc docs updated accordingly at https://github.com/KomodoPlatform/developer-docs/pull/395/files