external auth proto

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*Title*: *generate source code for external auth with protoc*

*Description*:
>I want to generate c# source code for the external auth service with protoc.
Any try failed with missing ... exceptions. Any how to would be nice.

[what have i done]
>clone this repo + googleapis + udpa
then in the envoy folder execute this
protoc --grpc_out=. --csharp_out=. -I=./api -I=../udpa -I=../googleapis -I=./envoy ./api/envoy/service/auth/v3/external_auth.proto

[exceptions]
>validate/validate.proto: File not found.
envoy/config/core/v3/socket_option.proto:7:1: Import "validate/validate.proto" was not found or had errors.
envoy/config/core/v3/address.proto:5:1: Import "envoy/config/core/v3/socket_option.proto" was not found or had errors.
envoy/config/core/v3/address.proto:12:1: Import "validate/validate.proto" was not found or had errors.
envoy/config/core/v3/address.proto:130:3: "SocketOptionsOverride" is not defined.
envoy/config/core/v3/address.proto:151:12: "SocketOption" is not defined.
envoy/config/core/v3/backoff.proto:9:1: Import "validate/validate.proto" was not found or had errors.
envoy/config/core/v3/http_uri.proto:9:1: Import "validate/validate.proto" was not found or had errors.
envoy/type/v3/percent.proto:7:1: Import "validate/validate.proto" was not found or had errors.
envoy/config/core/v3/base.proto:5:1: Import "envoy/config/core/v3/address.proto" was not found or had errors.
envoy/config/core/v3/base.proto:6:1: Import "envoy/config/core/v3/backoff.proto" was not found or had errors.
envoy/config/core/v3/base.proto:7:1: Import "envoy/config/core/v3/http_uri.proto" was not found or had errors.
envoy/config/core/v3/base.proto:8:1: Import "envoy/type/v3/percent.proto" was not found or had errors.
envoy/config/core/v3/base.proto:21:1: Import "validate/validate.proto" was not found or had errors.
envoy/config/core/v3/base.proto:213:12: "Address" is not defined.
envoy/config/core/v3/base.proto:273:3: "type.v3.Percent" is resolved to "envoy.type.v3.Percent", which is not defined. The innermost scope is searched first in name resolution. Consider using a leading '.'(i.e., ".type.v3.Percent") to start from the outermost scope.
envoy/config/core/v3/base.proto:422:3: "BackoffStrategy" is not defined.
envoy/config/core/v3/base.proto:435:3: "HttpUri" is not defined.
envoy/config/core/v3/base.proto:496:3: "type.v3.FractionalPercent" is resolved to "envoy.type.v3.FractionalPercent", which is not defined. The innermost scope is searched first in name resolution. Consider using a leading '.'(i.e., ".type.v3.FractionalPercent") to start from the outermost scope.
envoy/service/auth/v3/attribute_context.proto:5:1: Import "envoy/config/core/v3/address.proto" was not found or had errors.
envoy/service/auth/v3/attribute_context.proto:6:1: Import "envoy/config/core/v3/base.proto" was not found or had errors.
envoy/service/auth/v3/attribute_context.proto:57:5: "config.core.v3.Address" is not defined.
envoy/service/auth/v3/attribute_context.proto:184:3: "config.core.v3.Metadata" is not defined.
envoy/type/v3/http_status.proto:7:1: Import "validate/validate.proto" was not found or had errors.
envoy/service/auth/v3/external_auth.proto:5:1: Import "envoy/config/core/v3/base.proto" was not found or had errors.
envoy/service/auth/v3/external_auth.proto:6:1: Import "envoy/service/auth/v3/attribute_context.proto" was not found or had errors.
envoy/service/auth/v3/external_auth.proto:7:1: Import "envoy/type/v3/http_status.proto" was not found or had errors.
envoy/service/auth/v3/external_auth.proto:40:3: "AttributeContext" is not defined.
envoy/service/auth/v3/external_auth.proto:50:3: "envoy.type" seems to be defined in "envoy/type/v3/semantic_version.proto", which is not imported by "envoy/service/auth/v3/external_auth.proto". To use it here, please add the necessary import.
envoy/service/auth/v3/external_auth.proto:55:12: "config.core.v3.HeaderValueOption" is not defined.
envoy/service/auth/v3/external_auth.proto:75:12: "config.core.v3.HeaderValueOption" is not defined.
envoy/service/auth/v3/external_auth.proto:104:12: "config.core.v3.HeaderValueOption" is not defined.
envoy/service/auth/v3/external_auth.proto:108:12: "config.core.v3.QueryParameter" is not defined.

[optional *Relevant Links*:]
>Any extra documentation required to understand the issue.