Add support for implicit tag if struct field not explicitly marked required/optional

[lucasmoten]

• Normalizes as Required for field name vs Optional
• Adds tag 'implicit' when not declared.
• Don't error when decoding if a required field is implicit.

Reference: https://thrift.apache.org/docs/idl#field-requiredness

optional

• Write: Optional fields are only written when they are set
• Read: Optional fields may, or may not be part of the input stream.
• Default values: are written when the isset flag is set

Most language implementations use the recommended practice of so-called "isset" flags to indicate whether a particular optional field is set or not. Only fields with this flag set are written, and conversely the flag is only set when a field value has been read from the input stream.