decode: retain incompatible wire types as unknown fields

[vladislav-miroshnikov]

When a known field shows up with an incompatible wire type, generated decoding currently fails. DynamicMessage already treats such records as unknown fields, and this makes generated code do the same. A well-formed record goes through the unknown-field path and is kept or dropped depending on the message config. An earlier value of a singular or oneof field is kept, and malformed input still errors.

This covers owned, eager, lazy and table decoding. Most of the diff is regenerated checked-in code.

I left key/value records inside map entries alone, so they keep the strict checks from #387.

Regression tests cover kept values, defaults, unknown fields, packed and unpacked input, and malformed records. Lint, tests, arbitrary and docs pass on Linux.

Fixes #717.

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[vladislav-miroshnikov]

recheck