Propose to make j.t.f.DateTimePrintContext immutable

[wenshao]

I propose to make j.t.f.DateTimePrintContext immutable.
Currently, DateTimePrintContext has only one mutable field, optional. This can be replaced by adding an optional parameter to the DateTimeFormatter.formatTo method.
Immutable DateTimePrintContext can be optimized by escape analysis, such as immutable object optimization.
In order to make it easier for reviewers to see my ideas, I submitted a draft Pull Request ( https://github.com/openjdk/jdk/pull/26913 <https://github.com/openjdk/jdk/pull/26913 > ). Please give me feedback.
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