Feature request - HarmonyOptional attribute

[ManlyMarco]

Add a patch attribute that marks the patch as optional - if the method is not found or patching fails, the patch process is not aborted and there's a warning given, not an error.
It could have a parameter to specify if exceptions should be ignored or not. If target method is not found, it will always skip the patch (and show a short warning that an optional patch was skipped).

Use case is supporting multiple game versions where name or parameters of a method got changed.

[ManlyMarco]

Might require #52 to be fixed before an option to ignore exceptions can be added.

Could be combined with #62 into a single attribute with multiple options. e.g. [HarmonyOptional(OptionalMode.AllOverloads)]