Deprecate findCustom*Option functions in favor of extensions

[timostamm]

As a follow-up to #666, deprecate the findCustom*Option functions from @bufbuild/protoplugin/ecmascript.

Extensions provide a type-safe and more flexible way to read custom options in a plugin. Assuming that we have defined a custom service option:

extend google.protobuf.ServiceOptions {
  optional string default_host = 1001;
}

Previously, the option value could be read with a function call:

import { findCustomScalarOption } from "@bufbuild/protoplugin/ecmascript";

findCustomScalarOption(service, 1001, ScalarType.STRING)

The downside is that extension number and type must be provided by the user. With extensions, the generated extension object can be used instead, calling getExtension against the service options:

import { default_host } from "./customoptions/default_host_pb.js";
import { getExtension } from "@bufbuild/protobuf";

getExtension(service.proto.options, default_host);

The tutorial for writing plugins has been updated, and an example has been added to the plugin example.