Support for client-submitted transactions with ReCaptcha and allow-all validations

[sevazhidkov]

Currently, Octane doesn't support transaction that are submitted from client. Such transactions could be used to drain fee payer by requesting many transactions, breaking the state for them (for example, by withdrawing all of the tokens), then submitting them to the network with skip-preflight options. Fee payers get charged, but transaction fails.

For some fee payers, spending these fees could be tolerable and, basically, "cost of doing business". This PR introduces a non-default config option. When enabled, transfer and createAssociatedTokenAccount endpoints return signatures, instead of submitting transactions. buildWhirlpoolsSwap in this case will return pre-signed transaction, so sendWhirlpoolsSwap can be omitted.

The config option could be:

"returnSignature": {
    "type": "allowAll"
}

This way, all transactions will be allowed.

Alternatively, this config option will only pre-sign transactions if user passed reCaptcha validation:

"returnSignature": {
    "type": "reCaptcha",
    "reCaptchaProjectId": "",
    "reCaptchaSiteKey": "",
    "reCaptchaMinScore": 0.5
},

In this case, node operator has to set RECAPTCHA_API_KEY variable and pass reCaptchaToken with request.