Add validation for metrics, dimensions and namespace

[markkuhn]

Related issues:
#58

• Add validation for dimension values aws-embedded-metrics-node#131
• Add validation for metrics, dimensions and namespace aws-embedded-metrics-dotnet#35
• Add validation for metrics, dimensions and namespace aws-embedded-metrics-java#112

Instead of silently dropping non-compliant characters, the client will now throw an error if an invalid dimension is found according to specification here.