feat(log): Helper implemented io.Writer

[elricli]

Implemented io.Writer, easy to use in standard library, like log.New.

Description (what this PR does / why we need it):

Helper can set to http.Server{}.ErrorLog.

Example 1(transport/http):

// import stdlog "log"
// import log "github.com/go-kratos/kratos/v2/log"
// file: https://github.com/go-kratos/kratos/blob/main/transport/http/server.go#L153
srv.Server = &http.Server{
	Handler:   FilterChain(srv.filters...)(srv.router),
	// record http error log to helper.
	ErrorLog:  stdlog.New(srv.log.WithWriterLevel(log.LevelError), "", 0),
}

Example 2 (go-redis):

// import stdlog "log"
// import log "github.com/go-kratos/kratos/v2/log"
helper := log.NewHelper(logger, log.WithWriterLevel(log.LevelInfo))
redis.SetLogger(stdlog.New(helper, "", 0))

Other special notes for reviewer:

[codecov-commenter]

Codecov Report

Merging #1927 (bca4e54) into main (d5a2fb5) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1927      +/-   ##
==========================================
+ Coverage   80.65%   80.77%   +0.11%     
==========================================
  Files          81       82       +1     
  Lines        3587     3635      +48     
==========================================
+ Hits         2893     2936      +43     
- Misses        487      492       +5     
  Partials      207      207              

Impacted Files	Coverage Δ
log/helper_writer.go	100.00% <100.00%> (ø)
selector/default_node.go	91.66% <0.00%> (-8.34%)	⬇️
transport/grpc/resolver/discovery/builder.go	80.76% <0.00%> (-4.34%)	⬇️
errors/errors.go	94.44% <0.00%> (-1.79%)	⬇️
transport/http/client.go	78.88% <0.00%> (ø)
transport/http/status/status.go	100.00% <0.00%> (ø)
transport/grpc/resolver/discovery/resolver.go	76.78% <0.00%> (+0.42%)	⬆️
transport/http/codec.go	63.41% <0.00%> (+2.88%)	⬆️
transport/http/router.go	100.00% <0.00%> (+4.76%)	⬆️

---

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[tonybase]

infoWriter := log.NewWriter(logger, log.LevelInfo, "log")
warnWriter := log.NewWriter(logger, log.LevelWarn, "log")
errorWriter := log.NewWriter(logger, log.LevelError, "log")

redis.SetLogger(log.New(infoWriter, "redis: ", log.LstdFlags|log.Lshortfile))
grpclog.SetLogger(grpclog.NewLoggerV2(infoWriter, warnWriter, errorWriter))

You can provide a log writer to output third-party logs

[elricli]

infoWriter := log.NewWriter(logger, log.LevelInfo, "log")

Create and use a new function: NewWriter(logger Logger, level Level, msgKey string) instead of use WithWriterLevel, is it?

[tonybase]

Yes, you can create a Writer Helper to do this.

[elricli]

Updated.