AcreSDK: current position (#233)

Closes: #149 
---

### SDK: Return current position

---

SDK exposes the current staker's balances with two values:
stBTC shares balance (Obtained from: stBTC.balanceOf(address))
tBTC assets balance (Obtained from: stBTC.assetsBalanceOf(address) -
depends on #221)

sdk/src/lib/contracts/index.ts

@@ -1,12 +1,15 @@
 import { TBTCDepositor } from "./tbtc-depositor"
+import { StBTC } from "./stbtc"
 
 export * from "./tbtc-depositor"
 export * from "./chain-identifier"
+export * from "./stbtc"
 export * from "./depositor-proxy"
 
 /**
  * Represents all contracts that allow interaction with the Acre network.
  */
 export type AcreContracts = {
   tbtcDepositor: TBTCDepositor
+  stBTC: StBTC
 }

sdk/src/lib/contracts/stbtc.ts

@@ -0,0 +1,15 @@
+import { ChainIdentifier } from "./chain-identifier"
+
+export interface StBTC {
+  /**
+   * @param identifier The generic chain identifier.
+   * @returns Value of the basis for calculating final BTC balance.
+   */
+  balanceOf(identifier: ChainIdentifier): Promise<bigint>
+
+  /**
+   * @param identifier The generic chain identifier.
+   * @returns Maximum withdraw value.
+   */
+  assetsBalanceOf(identifier: ChainIdentifier): Promise<bigint>
+}