wsclient closed network connection err check

[phanidevavarapu]

This PR is to fix the issue #163.
When we try to ReadMessage on a closed connection, will through net.ErrClosed error.

Tested this adding the following code
https://github.com/open-telemetry/opamp-go/blob/main/client/wsclient.go#L243
go func() { time.Sleep(5 * time.Second) c.Stop(ctx) }()

opamp-go/client/internal/wsreceiver.go

Line 65 in 06011e4

_, bytes, err := r.conn.ReadMessage()

time.Sleep(5 * time.Second)

This allowed to close connection while the receiveMessage is in progress.

[linux-foundation-easycla]

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[codecov]

Codecov Report

Patch coverage: 90.00% and project coverage change: +0.38 🎉

Comparison is base (efddaa2) 76.11% compared to head (d71e9fd) 76.49%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #172      +/-   ##
==========================================
+ Coverage   76.11%   76.49%   +0.38%     
==========================================
  Files          24       24              
  Lines        1834     1868      +34     
==========================================
+ Hits         1396     1429      +33     
- Misses        326      327       +1     
  Partials      112      112              

Impacted Files	Coverage Δ
client/types/callbacks.go	66.66% <ø> (ø)
server/httpconnection.go	33.33% <0.00%> (ø)
client/internal/mockserver.go	82.35% <85.71%> (+0.31%)	⬆️
server/serverimpl.go	58.92% <94.73%> (+0.37%)	⬆️
client/httpclient.go	95.00% <100.00%> (+0.26%)	⬆️
client/internal/httpsender.go	72.22% <100.00%> (+1.25%)	⬆️
client/internal/wsreceiver.go	91.89% <100.00%> (+0.46%)	⬆️
server/callbacks.go	68.18% <100.00%> (-4.55%)	⬇️
server/wsconnection.go	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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