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GH-34492: [Go] Fix missing boolean plain encoder state update (#34493)



### Rationale for this change

This is a bugfix in the go `PlainBooleanEncoder`. Statistics of boolean columns, as well as data in rare cases, can be malformed. This is a data-correctness bugfix.

### What changes are included in this PR?

`FlushValue` does not reset the intermediary BitMapWriter position. In this way previously-encoded values might "leak" into subsequent `FlushValue` calls. Page emission and statistics emission (for examples) manifest this issue.

### Are these changes tested?

Yes. The most direct test is the new `TestBooleanPlainDecoderAfterFlushing` in internal\encoding\encoding_test.go, but I also added `TestBooleanStatisticsEncoding` as a "consumer" of that encoder.

Question: should I add more testing? I experimentally harnessed a fuzz test targeted towards the Boolean encoder, and that manifested the issue, but that's a fuzz test and also targeted towards Bools alone. I also provisionally implemented a (hacky) extension to the roundtrip tests to have an intermediate flush, but that didn't seem enough to manifest the (somewhat value-count-sensitive) issue.

### Are there any user-facing changes?
This is a correctness fix, so other than that, no.

**This PR contains a "Critical Fix".**
* Closes: #34492

Authored-by: Aaron Gorenstein <aaron.gorenstein@mongodb.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>