Add int-type Authz ID proto fields alongside any string-type proto fields

[ezekiel]

This is stage one of normalizing Authz ID to be consistently int64-type (#8722).

[ezekiel]

Still adding real fixes for load-generator.

[jsha]

Looks good. Two comments:

You asked this morning about possible split points for future PRs like this. One natural split point I noticed: the load-generator changes. That could be a nice tidy separate PR refactoring the load-generator not to depend on core.Authorization. Not needed for this one but as an FYI.

Also, I noticed the if id != 0 { ... } else if idstr != "" { ... } else { ... } pattern in four places. That's enough to justify a shared helper:

// integerID returns `id` if it is non-zero, otherwise `idStr` parsed as an integer.
// Returns either if both args have their zero value.
func integerID(id int64, idStr string) (int64, error) {

[ezekiel]

Very helpful, thank you.