Add post actions to return wallet details

[farukterzioglu]

Add relevant methods to return address-private key pairs
e.g.

[HttpPost("/Avalanche/{account}/external/{index}")]
public ActionResult<KeyPair> GetDepositWallet(uint account, uint index)

For only apps that allowed this by configuration exposePrivateKey

should return a response like;

{
 "address": "X-avax1....",
 "privateKey" : "adf54djmr...."
}