auction contract example opt_in method doesn't have opt_in allow actions

[iskysun96]

Subject of the issue

puya/examples/auction/contract.py

Line 62 in 26e66e7

@arc4.abimethod

The opt_in method should have the allow_actions=['OptIn']. Without it, app caller won't be opted in to the contract.

Change code from:

@arc4.abimethod()
def opt_in(self) -> None:
    pass

to

    @arc4.abimethod(allow_actions=["OptIn"])
    def opt_in(self) -> None:
        pass

[robdmoore]

I suspect there's a number of problems with the auction example, it's been copied down from an original one (in beaker?) that was half baked across a few programming languages haha.

Feel free to submit a pull request to resolve this one.

[iskysun96]

@robdmoore Could you review this PR? I also have a couple questions mentioned in the PR #311