feature: poolable messages paths with wildcard

[evgfedotov]

There are currently two ways to define messages to which the "pool" functionality will be applied:

1. Via per-message optionvtproto.mempool
2. By enumerating messages with the pool flag

Both of these methods uncomfortable to use for large numbers of messages.

It might be worth adding the ability to use wildcards in pool flag to determine group of messages.

For example in my existing buf configuration:

version: v1
plugins:
    - name: go-vtproto
      path: bin/protoc-gen-go-vtproto
      out: .
      opt:
        - features=marshal+unmarshal+size+pool+grpc
        - paths=source_relative
        - pool=github.com/some/api/example1.Message1
        - pool=github.com/some/api/example2.Message2
        - pool=...
        - pool=github.com/some/api/example3.Message100
      strategy: directory

Instead could be:

version: v1
plugins:
    - name: go-vtproto
      path: bin/protoc-gen-go-vtproto
      out: .
      opt:
        - features=marshal+unmarshal+size+pool+grpc
        - paths=source_relative
        - pool=github.com/some/service/api/*
      strategy: directory

For more flexibility it is also possible to add a flag to exclude some messages from pool feature generation (something like pool-exclude).

[evgfedotov]

@vmg, Hi!

I would be very grateful if you could watch this 😃

[vmg]

I followed up on #115 just this morning!

[tristan-instacart]

Worthy of note for those looking to use the wildcards, the following syntax does not work:
pool=github.com/some/service/api/*

The final element of the path is the . to get the message not a slash, so this following should work:
pool=github.com/some/service/api.*

In my case I have proto files that look like this
service/v2/file.proto
service/v2/file2.proto
service/v2/file3.proto

with a go package that looks like
option go_package = "github.com/service/v2;servicepb"

if I specify pool=service/v2/* I get nothing, if I specify pool=github.com/service/v2.* it works correctly

I tried using the ** as well and it didn't resolve, I needed to use .* off the package name.

I opened this to add to readme #155