Move extension type construction logic out of Field

[scovich]

Which issue does this PR close?

• Part of [arrow-json] deserialize Variant fields #8987

Rationale for this change

The logic to instantiate a type extension does not really depend on Field, other than indirectly because that struct happens to contain all the necessary bits of information.

As part of the work to make the JSON decoder support extension types, it was observed that a field is not always available (or at least, not desirable because it creates redundancy). This change addresses the concern by making it possible to work directly with extension types instead of being forced to route through a Field instance.

What changes are included in this PR?

Factor out the body of Field::try_extension_type as a new associated function ExtensionType::try_new_from_field_metadata that takes data type and field metadata map and delegates to ExtensionType::try_new. Field::try_extension_type then simply calls that new method.

Are these changes tested?

Code movement. Existing unit tests validate it.

Are there any user-facing changes?

New provided trait method.

[alamb]

Thanks @scovich -- this makes sense to me

[scovich]

@alamb -- any reason this shouldn't merge?