Retrun initial deposit amount from _initializeDeposit function

The amount may be useful for integrators for validations of the deposit
amount. See example use case in thesis/acre#253 (comment)

It turns out that this version where we read from the storage costs
`117 366` gas. Which is less than an alternative approach:
```
        initialDepositAmount =
            fundingTx
                .outputVector
                .extractOutputAtIndex(reveal.fundingOutputIndex)
                .extractValue() *
            SATOSHI_MULTIPLIER;
```
which costs `117 601` gas.

solidity/contracts/integrator/AbstractTBTCDepositor.sol

@@ -123,6 +123,8 @@ abstract contract AbstractTBTCDepositor {
     ///         `keccak256(fundingTxHash | reveal.fundingOutputIndex)`. This
     ///         key can be used to refer to the deposit in the Bridge and
     ///         TBTCVault contracts.
+    /// @return initialDepositAmount Amount of funding transaction deposit. In
+    ///         TBTC token decimals precision.
     /// @dev Requirements:
     ///      - The revealed vault address must match the TBTCVault address,
     ///      - All requirements from {Bridge#revealDepositWithExtraData}
@@ -134,10 +136,10 @@ abstract contract AbstractTBTCDepositor {
         IBridgeTypes.BitcoinTxInfo calldata fundingTx,
         IBridgeTypes.DepositRevealInfo calldata reveal,
         bytes32 extraData
-    ) internal returns (uint256) {
+    ) internal returns (uint256 depositKey, uint256 initialDepositAmount) {
         require(reveal.vault == address(tbtcVault), "Vault address mismatch");
 
-        uint256 depositKey = _calculateDepositKey(
+        depositKey = _calculateDepositKey(
             _calculateBitcoinTxHash(fundingTx),
             reveal.fundingOutputIndex
         );
@@ -148,7 +150,9 @@ abstract contract AbstractTBTCDepositor {
         // an explicit check here.
         bridge.revealDepositWithExtraData(fundingTx, reveal, extraData);
 
-        return depositKey;
+        initialDepositAmount =
+            bridge.deposits(depositKey).amount *
+            SATOSHI_MULTIPLIER;
     }
 
     /// @notice Finalizes a deposit by calculating the amount of TBTC minted

solidity/contracts/test/TestTBTCDepositor.sol

@@ -11,7 +11,10 @@ import "../integrator/ITBTCVault.sol";
 import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
 
 contract TestTBTCDepositor is AbstractTBTCDepositor {
-    event InitializeDepositReturned(uint256 depositKey);
+    event InitializeDepositReturned(
+        uint256 depositKey,
+        uint256 initialDepositAmount
+    );
 
     event FinalizeDepositReturned(
         uint256 initialDepositAmount,
@@ -28,8 +31,12 @@ contract TestTBTCDepositor is AbstractTBTCDepositor {
         IBridgeTypes.DepositRevealInfo calldata reveal,
         bytes32 extraData
     ) external {
-        uint256 depositKey = _initializeDeposit(fundingTx, reveal, extraData);
-        emit InitializeDepositReturned(depositKey);
+        (uint256 depositKey, uint256 initialDepositAmount) = _initializeDeposit(
+            fundingTx,
+            reveal,
+            extraData
+        );
+        emit InitializeDepositReturned(depositKey, initialDepositAmount);
     }
 
     function finalizeDepositPublic(uint256 depositKey) external {

solidity/test/integrator/AbstractTBTCDepositor.test.ts

@@ -108,6 +108,8 @@ describe("AbstractTBTCDepositor", () => {
       })
 
       context("when deposit is accepted by the Bridge", () => {
+        const expectedInitialDepositAmount = to1ePrecision(10000, 10)
+
         let tx: ContractTransaction
 
         before(async () => {
@@ -133,7 +135,7 @@ describe("AbstractTBTCDepositor", () => {
         it("should return proper values", async () => {
           await expect(tx)
             .to.emit(depositor, "InitializeDepositReturned")
-            .withArgs(fixture.expectedDepositKey)
+            .withArgs(fixture.expectedDepositKey, expectedInitialDepositAmount)
         })
       })
     })