New, more flexible try-runtime runtime apis

[liamaharon]

Existing try-runtime apis accept little to no configuration.

This means when we want to change the behavior of calling one of these apis, it requires adding complexity to the runtime, and in some cases waiting for new runtime upgrades to occur.

e.g.

• Configuration of which try-state checks to run should be configurable by the user, instead of us getting stuck with inoperable checks until the next runtime upgrade
• Instead of Skip System::Events in TryDecodeEntireState #2560 requiring a new pallet macro, the user should be able to pass a whilelist and/or blacklist of storage items they want to attempt to decode

This is less than ideal. It would be better if runtime apis allowed greater customisation of what happens. Once added, old runtime apis should be deprecated and eventually removed.

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also see discussion at #2108