Minor(proto): Implement TryFrom<&DFSchema> for protobuf::DfSchema

[jonahgao]

Which issue does this PR close?

N/A

Rationale for this change

Currently, we have only implemented TryFrom<&DFSchemaRef> for protobuf::DfSchema.
In the example below, when encoding a DFSchema, we have to create an Arc, which can be considered wasteful.

let dfschema = datafusion::common::DFSchema::empty();
let arc_dfschema = Arc::new(dfschema);
let proto_dfschema: datafusion_proto::DfSchema =(&arc_dfschema).try_into().unwrap();

Implementing TryFrom<&DFSchema> for protobuf::DfSchema helps to avoid this issue,
and then the code is simplified as follows.

let dfschema = datafusion::common::DFSchema::empty();
let proto_dfschema: datafusion_proto::DfSchema =(&dfschema).try_into().unwrap();

What changes are included in this PR?

Are these changes tested?

Yes

Are there any user-facing changes?

New API, without breaking changes

[alamb]

Thanks @jonahgao