jsonpb: decode int32/uint32/float32/float64 strings

[kassiansun]

According to
https://developers.google.com/protocol-buffers/docs/proto3#json,
all integers & floats are accepted as strings.

We still preserve the encoding behavior, to parse int64s to strings and other number to number.

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

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What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
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• The email used to register you as an authorized contributor must also be attached to your GitHub account.

[kassiansun]

I signed it!

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[kassiansun]

Anyone'd like to take a look at this?

[dsnet]

Looks good. Thank you.