Fix ClientSim compatibility with SDK 3.6.1

[FxckingAngel]

Summary

• update UdonProductStub to implement the current IProduct.Purchase overload
• use string event names for ClientSim events that are not exposed as UdonManager constants in SDK 3.6.1

Validation

Tested in a clean world project using Unity 2022.3.22f1 and VRChat SDK 3.6.1.

Tundra build success (19.17 seconds), 13 items updated, 705 evaluated
Exiting without the bug reporter. Application will terminate with return code 0

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[FxckingAngel]

I have read the CLA Document and I hereby sign the CLA

[Faxmashine]

Thanks for taking the time to put this together, and for signing the CLA.

We unfortunately can't accept this PR. Both changes look like
they're working around SDK 3.6.1 specifically (rather than fixing something in
ClientSim itself):

• UdonManager.UDON_EVENT_ONLANGUAGECHANGED and
  UdonManager.UDON_EVENT_ONVRCPLUSMASSGIFT are still present and public in
  current SDK versions, so swapping them for string literals would be a
  regression. We'd lose the compile-time safety of the constants on supported SDKs.
• The Purchase() overload added to UdonProductStub doesn't match the
  signature on the current IProduct interface, so it wouldn't compile against
  a current SDK.

SDK 3.6.1 was released about two years ago
(https://creators.vrchat.com/releases/release-3-6-1), and ClientSim tracks the
current SDK rather than maintaining compatibility with older releases. Our
recommendation would be to update your project's SDK, which should resolve the
compile errors you're encountering.

If you update and still run into problems with ClientSim, please open a new
issue with the details.