Makes AesGcm Sample Thread Safe

[joelrozen]

What was changed

Changed Encryption Codec in Encryption Sample to use a new instance of AesGcm on each call.

Why?

See #50

Checklist

1. Closes [Bug] AesGcm instance should not be reused in EncryptionCodec #50

2. How was this tested:

Ran several thousand simultaneous workflows

3. Any docs updates needed? No

[CLAassistant]

All committers have signed the CLA.

[cretz]

Thanks! A couple of suggestions but otherwise looks good.

[cretz]

Looks good! Just a compile error to fix.

[cretz]

CI still not passing, a couple of typos. Run dotnet build to confirm it builds.

[joelrozen]

[cretz]

Thanks!