AIP-231 should clarify whether it permits batch-getting top-level resources

[apasel422]

And if so, whether that implies that the parent field should be omitted from the request and the URI pattern.

If that's the case, we should also update the linter, as it appears to have false positives around things like

rpc BatchGetWidgets(BatchGetWidgetsRequest) returns (BatchGetWidgetsResponse) {
    option (google.api.http) = {
      get: "/v2/widgets:batchGet"
    };
}

as the GET URI doesn't match the regex {[a-zA-Z=/*]+}/[A-Za-z0-9-=/*]+:batchGet$ at https://github.com/googleapis/api-linter/blob/c6ca8351663b4679ba5a8c571dfede4ff4d22228/rules/aip0231/aip0231.go#L45.

Other linters that need to be updated if so are:

• https://linter.aip.dev/136/http-uri-suffix
• https://linter.aip.dev/231/request-parent-field
• https://linter.aip.dev/233/http-uri-suffix
• https://linter.aip.dev/233/request-parent-field
• https://linter.aip.dev/234/http-uri-suffix
• https://linter.aip.dev/234/request-parent-field