[foreign-preview] RFR: 8281412: MemorySegment::map should live in FileChannel

[Julia Boes]

This change moves the factory for mapped memory segments to `FileChannel`, as a new instance method `map`, which moves it closer to the existing `FileChannel::map` that returns a `MappedByteBuffer`.  

Tests are adjusted accordingly and a new test for custom file channels (that do/don't override the method) is added.

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Commit messages:
 - let caller close the file channel
 - update docs and tests and add custom file channel test

Changes: https://git.openjdk.java.net/panama-foreign/pull/668/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=668&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281412
  Stats: 465 lines in 10 files changed: 308 ins; 110 del; 47 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/668.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/668/head:pull/668

PR: https://git.openjdk.java.net/panama-foreign/pull/668