[Housekeeping] Explore reenabling protoc-gen-validate

[katrogan]

Describe the issue

#4643 removed this, but flyteadmin validation code checks a ton of request fields for whether they're empty https://github.com/search?q=repo%3Aflyteorg%2Fflyte%20ValidateEmptyStringField&type=code

this manual code (and other checks) could be moved to the pb layer and invoked as middleware to automate these checks

What if we do not do this?

more hand-curated code, easy to miss adding for new apis

Related component(s)

flyteidl, flyteadmin

Are you sure this issue hasn't been raised already?

• Yes

Have you read the Code of Conduct?

• Yes

[katrogan]

cc @eapolinario