Add "format" property to google.protobuf.Duration string

[grant]

google.protobuf.Duration types should has a format / pattern property with the expected formats that are valid for this string.

Ref:

• https://github.com/chrusty/protoc-gen-jsonschema/issues/63
• https://json-schema.org/understanding-json-schema/reference/string.html

Examples:

• "3.000001s"
• "3s"

[chrusty]

A PR is open for this: https://github.com/chrusty/protoc-gen-jsonschema/pull/67

[chrusty]

I've merged a fix for this, and made a pre-release here: https://github.com/chrusty/protoc-gen-jsonschema/releases/tag/untagged-f8dda646aa02d3927db9