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GH-36950: [C++] Change std::vector<std::shared_ptr<Field>> to use it's alias: FieldVector (#37101)

### Rationale for this change

Clarity and Maintainability

### What changes are included in this PR?

1. Changed all occurrence of std::vector<std::shared_ptr<Field>> in **type_fwd.h**、**type.h** and **type.cc** to FieldVector for consistency.
2. Use move to eliminate vector copy in sparse and dense union's constructor

### Are these changes tested?

Covered by existing tests

### Are there any user-facing changes?

No

* Closes: #36950

Authored-by: jsjtxietian <jsjtxietian@outlook.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>