[foreign-jextract] RFR: 8261032: jextract should have an option to specify main header class name

[Athijegannathan Sundararajan]

adding --header-class-name option

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Commit messages:
 - 8261032: jextract should have an option to specify main header class name

Changes: https://git.openjdk.java.net/panama-foreign/pull/496/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=496&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261032
  Stats: 24 lines in 3 files changed: 23 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/496.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/496/head:pull/496

PR: https://git.openjdk.java.net/panama-foreign/pull/496