Minor: Add Extensions::new()

[alamb]

Which issue does this PR close?

N/A

Rationale for this change

@andygrove reported on [ASF Slack](https://the-asf.slack.com/archives/C01QUFS30TD/p1679408811522469

I am trying to upgrade Ray SQL to DataFusion 20, and running into problems trying to create the new Extensions param to TaskContext::try_new
The struct is defined as:
#[derive(Debug, Default, Clone)]
pub struct Extensions(BTreeMap<&'static str, ExtensionBox>);

I am hitting this error:
error[E0423]: cannot initialize a tuple struct which contains private fields
--> src/context.rs:177:13
|
177 | Extensions(BTreeMap::new())
| ^^^^^^^^^^
|
note: constructor is not visible here due to private fields

Am I missing something obvious here, or is it no longer possible to create TaskContext outside of the core project?

Turns out the answer was Extensions::default:

11:10 AM
ah, nm, ... `
Extensions::default()

What changes are included in this PR?

Add an Extensions::new() which is the idiomatic way to create "new" objects in Rust

Are these changes tested?

Yes -- updated existing tests

Are there any user-facing changes?

Slightly better ergonomics and discoverability

[andygrove]

Thanks @alamb. That is certainly easier to find.