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GH-43543: [FlightRPC][C++] Reduce the number of references to protobuf::Any (#43544)

### Rationale for this change

The less code depends on `protobuf::Any`, the easier it is to separate the parts that need to link against pb libraries from the ones that can be built without it.

### What changes are included in this PR?

 - Internal forward declarations (plus making some dtor/ctor non-inline)
 - Add missing `SchemaResult::FromProto`
 - Extract a few proto-related functions
 - Move more generic code from `flight/sql` to `sql`

### Are these changes tested?

By existing tests.
* GitHub Issue: #43543

Authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Signed-off-by: Felipe Oliveira Carvalho <felipekde@gmail.com>