REGTESTS: protobuf: cover all the field types of the ungrpc converter

REGTESTS: protobuf: cover all the field types of the ungrpc converter

The first message only used int32 leaves and a string, that is two of
the fifteen types the converter accepts as its second argument, and a
single gRPC message, while the converter is able to walk over as many
of them as the buffer holds.

A second frontend receives a message holding one field per type, the
field number being the same as the index of the type in the list of the
documentation, so that every branch of protobuf_smp_store_varint(),
protobuf_smp_store_32bit() and protobuf_smp_store_64bit() is visited,
including the zigzag decoding of sint32/sint64 and the little-endian
decoding of the fixed size types. Two fields are also extracted without
a type to check the binary form, and three others with a type which
does not match their wire type, which must return nothing.

The test records that a "float" or a "double" field is truncated to an
integer, which is what the documentation announces ("as an integer
sample") but is easy to overlook.

A third frontend receives a body made of two gRPC messages so that the
loop searching the field in the messages which follow is covered.