timestamp does not allow milliseconds in ISO date strings

[gregfitzpatrick1]

While looking at the JSON schema files used for validation, I noticed that the regex pattern for timestamp in command.schema.json fails if milliseconds are included in the timestamp.

Looking at the OCPI 2.1.1 documentation on DateTime type, I see that milliseconds are not shown in the examples, but claims to use the ISO 8601 standard. Looking at the ISO 8601 description, it looks like milliseconds should be allowed optionally.

I've seen this confusion in some implementations of OCPI. Unless I'm the one who is confused, I believe timestamp should be updated in these JSON schemas to allow for milliseconds optionally, and the OCPI documentation made more clear on this.