Adding an extension number for protoc-gen-jsonschema

[chrusty]

Some users of my project have requested features which will require custom field options. I've picked some high random ones for now, but it would be good to do this the proper way! I hope you have plenty of address space here.

[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to determine that you authored the commits in this PR. Maybe you used a different email address in the git commits than was used to sign the CLA? If someone else authored these commits, then please add them to this pull request and have them confirm that they're okay with them being contributed to Google. If there are co-authors, make sure they're formatted properly.

In order to pass this check, please resolve this problem and then comment@googlebot I fixed it... If the bot doesn't comment, it means it doesn't think anything has changed.

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[acozzette]

@chrusty I think the CLA bot is giving us an error because your email address is not set on the commit. Would you mind amending the commit to set the same email address you used to sign the CLA?

[chrusty]

@acozzette hey, I updated and the cla-bot has given my PR a green label. Now it looks like I need some other labels to be added before this can be approved / merged, but I guess that is up to you.

Thanks for looking at this!

[chrusty]

Awesome, thanks!