Validate that ASCII Metadata values contain only printable ASCII Characters

[CoolTomatos]

To tackle grpc/grpc-go#468, in grpc/grpc-go#4886, a stricter validation on ASCII Metadata values was introduced to throw if the value contains any non-printable ASCII characters, that is characters outside of the range 0x20 to 0x7E.

The current grpc-java behavior regarding that is:

• When putting entries to a Metadata instance:
  • Any non ASCII character is silently replaced with ?;
  • Any ASCII character is kept and marshaled as it is, this includes non-printable ones like \t for example.
• When sending the Metadata instance:
  • Entires where the value contains non-printable ASCII are logged and dropped.

This pull request fixes the inconsistent behavior among grpc-java and grpc-go and closes #11679.

Notice that with this pull request, previously passing ASCII Metadata values, will now observe IllegalArgumentExceptions therefore this is a breaking change.

[linux-foundation-easycla]

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