Expose an option to reject unknown JSON members

[negz]

Description of your changes

Closes #121

@phisco This is how I would wrap JSON options in our own, to avoid coupling ourselves to the upstream (and not yet stable) json-experiment implementation too strongly.

I'm opening as a draft though because after writing this it occurred to me that I'd prefer to make RejectUnknownMembers the default behavior for AsObject. My rationale:

• I think it's the safest/least surprising in our use-case.
• I believe it matches the default behavior of protojson.Marshal (i.e. AsStruct).
• It's a breaking behavior change, but...
  • My hunch is it won't break anyone in practice yet.
  • We're still at v0.x and the first release, so breaking changes are acceptable when warranted.
• We can always add an option in future if there's a demand to turn it off.

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable to ensure this PR is ready for review.

How has this code been tested

[phisco]

Makes sense to me making it the default behavior, should we add options to all functions ending up in a AsObject being called?

[negz]

Makes sense to me making it the default behavior, should we add options to all functions ending up in a AsObject being called?

I think if we're okay making this the default behavior we can defer adding options at all for the time being.