Added mutex to avoid concurrency errors on SubscriptionManager

[danielbdias]

This PR aims to fix a concurrency error that is happening on Tracetest:

persistentRunner job. ID 1, testID JFxVPFY4R, TraceID 8ec5afeaca8d38668e5053fc4fb79998, SpanID b8a543d68fd34165
fatal error: concurrent map read and map write

goroutine 49 [running]:
github.com/kubeshop/tracetest/server/subscription.(*Manager).Publish(0x1a97540?, {0xc002b95740, 0x1a}, {0x1a97540, 0xc0033331e0})
	/home/runner/work/tracetest/tracetest/server/subscription/manager.go:59 +0x65
github.com/kubeshop/tracetest/server/executor.(*internalEventEmitter).Emit(_, {_, _}, {0x0, {0x1b2908b, 0xc}, {0x1b1a196, 0x7}, {0x1b5697d, 0x1c}, ...})
	/home/runner/work/tracetest/tracetest/server/executor/eventemitter.go:35 +0x18d
github.com/kubeshop/tracetest/server/executor.persistentRunner.processExecQueue({0xc0006b7fb0, {0x7f86d6a14208, 0xc000522e40}, {0x7f86d6a141b8, 0xc000136068}, {0x1f32940, 0xc000131410}, {0x1f2fda0, 0xc000092940}, 0xc0002a2050, ...}, ...)
	/home/runner/work/tracetest/tracetest/server/executor/runner.go:177 +0x262
github.com/kubeshop/tracetest/server/executor.persistentRunner.Start.func1()
	/home/runner/work/tracetest/tracetest/server/executor/runner.go:115 +0x238
created by github.com/kubeshop/tracetest/server/executor.persistentRunner.Start
	/home/runner/work/tracetest/tracetest/server/executor/runner.go:100 +0x85

Changes

Fixes

Checklist

• tested locally
• added new dependencies
• updated the docs
• added a test

[schoren]

LGTM!

[danielbdias]

Added this dependency to avoid running tests based on build if our unit tests break by some reason.