[middleware] Add protobuf binding middleware

[FumingPower3925]

Context

Protocol Buffers serialization removed from core to avoid forcing google.golang.org/protobuf as a transitive dependency.

Scope

Create a protobuf middleware package that provides:

• ProtoBuf(c *celeris.Context, code int, v proto.Message) error
• BindProtoBuf(c *celeris.Context, v proto.Message) error
• Bind() helper that auto-detects application/protobuf and application/x-protobuf

Dependencies

• google.golang.org/protobuf

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Migrated from goceleris/middlewares#33