Protoc-gen-validate

Changes
- update golang/protobuf to v1.4.2
- update lyft/protoc-gen-star to v0.5.1
- update google/re2 to 2020-07-06
- fix hostname validation in python
- fix timestamp comparison for gt_now and within
- cache generated functions for python validation to improve perf for repeated messages

Changes
=============
- removes support for Gogo proto
- use Go modules instead of vendored dependencies
- fixes Go generation for repeated enum with `not_in` constraint (329)
- fix build to work with `bazelisk --strict` (327)

v0.3.0-java
Fixed:
* Only add a generater java file if lang == "java" 242
* Append correct literal suffix for map keys/values constraints 294
* Update Maven Central server URLs to use HTTPS 311

Added:
* Java validation errors now included proto-encoded field details. 317
* Add new validation type `not_contains` 253

v0.2.0-java
Fixed:
* Correctly handle messages with multiple consecutive uppercase characters 204
* `ReflectiveValidatorIndex` correctly handles `java_multiple_files=true` 208
* Correctly handle messages with underscores when ``java_multiple_files=true` 233

Added:
* Can now provide fallback validator implementations for when reflection doesn't find a validator 230