Get include-what-you-use work for Envoy

Title: Get include-what-you-use work for Envoy

Description:
There are lots of include-what-you-use violations in Envoy:
1. over-inclusion: including too many unused headers slows down compilation.
2. transitive-inclusion: some of the missing explicit includes will make further development difficult.

[include-what-you-use](https://include-what-you-use.org/) is powerful to address those violations and fix them.

@htuch mentioned at #10917 that we need to get it work for Envoy.

I tried on my own development environment and it works.

Here are steps to get it work:

1. follow instructions at [Instructions for developers] (https://github.com/include-what-you-use/include-what-you-use/blob/master/README.md) to build iwyu
2. generate compilation database by
```
tools/gen_compilation_database.py --include_headers
```
3. replace `-stdlib=libc++` `-stdlib=libstdc++` in the generated json file `compile_commands.json`

4. run ` iwyu_tool.py -p .` at Envoy root directory. (Please refer to section "Using with a compilation database" from [Instructions for developers] (https://github.com/include-what-you-use/include-what-you-use/blob/master/README.md) for details)

The output shall look like this

```
...
source/common/grpc/google_async_client_impl.cc should add these lines:
#include // for GPR_CLOCK_REALTIME
#include // for gpr_inf_future
#include // for Channel
#include // for ClientAsyncReader...
#include // for WriteOptions
#include // for string_ref
#include // for duration
#include // for tie, tuple
#include "absl/strings/match.h" // for EndsWith
#include "common/grpc/stat_names.h" // for StatNames
#include "common/http/header_map_impl.h" // for RequestHeaderMapImpl
#include "common/protobuf/utility.h" // for PROTOBUF_GET_WRAP...
#include "common/stats/symbol_table_impl.h" // for StatName
#include "envoy/api/api.h" // for Api
#include "envoy/common/time.h" // for TimeSource
#include "envoy/stats/stats.h" // for Counter

source/common/grpc/google_async_client_impl.cc should remove these lines:
- #include "common/config/datasource.h" // lines 9-9
- #include "common/grpc/common.h" // lines 10-10
- #include "common/grpc/google_grpc_creds_impl.h" // lines 11-11
- #include "grpcpp/support/proto_buffer_reader.h" // lines 15-15

The full include-list for source/common/grpc/google_async_client_impl.cc:
#include "common/grpc/google_async_client_impl.h"
#include // for GPR_CLOCK_REALTIME
#include // for gpr_inf_future
#include // for Channel
#include // for ClientAsyncReader...
#include // for WriteOptions
#include // for string_ref
...
```

It will be useful if we can apply the iwyu check to every pull request to check iwyu violations.