SNIP-36 - helper for creation of signed transaction

[PhilippeR26]

Motivation and Resolution

In the frame of SNIP-36, proof are generated by an off-line sequencer backend. A RPC.INVOKE_TXN_V3 signed transaction has to be sent to this backend.
Today, Account.execute is generating this RPC.INVOKE_TXN_V3 internally and send it to the node, but we don't have a method to provide these data to users.
This PR is adding a such method.

Usage related changes

• Added a new Account.getSignedTransaction() method. Parameters are 100% the same than Account.execute(). Example:

const myCall = myTestContract.populate("Say_HelloPhil126",  { messages: "0x123" });
const tx3: INVOKE_TXN_V3 = await account0.getSignedTransaction(myCall);

Then tx3 can be sent to the backend, to generate the proof.

• Added a new RpcProvider.invokeSignedTx(). This method is not necessary in the frame of SNIP-36, but it's consistent to have in Starknet.js both a method to create a signed Tx and a method to consume it. Example:

const res = await account0.provider.channel.invokeSignedTx(tx3);
const txR3 = await account0.provider.waitForTransaction(res.transaction_hash);

Development related changes

As 90% of code of Account.execute() is common with Account.getSignedTransaction(), this common part has been written is a new prepareInvoke() function.

Checklist:

• Performed a self-review of the code
• Rebased to the last commit of the target branch (or merged it into my branch)
• Documented the changes in code (API docs will be generated automatically)
• Updated the tests
• All tests are passing

[tabaktoni]

@copilot resolve the merge conflicts in this pull request