This commit creates the copyRestSpec task for rest integ tests
immediately on creation of the RestIntegTestTask instead of lazily in
afterEvaluate. This allows other projects to add additional rest specs
to be copied, instead of needing to create another parallel copy task.

* es/master: (50 commits)
  Reject updates to the `_default_` mapping. (#29165)
  Improve similarity docs. (#29089)
  [Docs] Update api.asciidoc (#29166)
  Docs: Add note about missing mapping for doc values field (#29036)
  Fix BWC issue for PreSyncedFlushResponse
  Remove BytesArray and BytesReference usage from XContentFactory (#29151)
  Add pluggable XContentBuilder writers and human readable writers (#29120)
  Add unreleased version 6.2.4 (#29171)
  Add unreleased version 6.1.5 (#29168)
  Add a note about using the `retry_failed` flag before accepting data loss (#29160)
  Fix typo in percolate-query.asciidoc (#29155)
  Require HTTP::Tiny 0.070 for release notes script
  Set Java 9 checkstyle to depend on checkstyle conf (#28383)
  REST high-level client: add clear cache API (#28866)
  Docs: Add example of resetting index setting (#29048)
  Plugins: Fix module name conflict check for meta plugins (#29146)
  Build: Fix meta plugin bundled plugin names (#29147)
  Build: Simplify rest spec hack configuration (#29149)
  Build: Fix meta modules to not install as plugin in tests (#29150)
  Fix javadoc warning in Strings for missing parameter description
  ...

* es/6.x: (46 commits)
  Docs: Add note about missing mapping for doc values field (#29036)
  [DOCS] Removed 6.1.4, 6.2.2, and 6.2.3 coming tags
  Remove BytesArray and BytesReference usage from XContentFactory (#29151)
  Fix BWC issue for PreSyncedFlushResponse
  Add pluggable XContentBuilder writers and human readable writers (#29120)
  Add unreleased version 6.2.4 (#29171)
  Add unreleased version 6.1.5 (#29168)
  Add a note about using the `retry_failed` flag before accepting data loss (#29160)
  Fix typo in percolate-query.asciidoc (#29155)
  Add release notes for 6.1.4 and 6.2.3
  Require HTTP::Tiny 0.070 for release notes script
  REST high-level client: add clear cache API (#28866)
  Relax remote check for bwc project checkouts (#28666)
  Set Java 9 checkstyle to depend on checkstyle conf (#28383)
  Docs: Add example of resetting index setting (#29048)
  Plugins: Fix module name conflict check for meta plugins (#29146)
  Build: Fix meta plugin bundled plugin names (#29147)
  Build: Simplify rest spec hack configuration (#29149)
  CLI: Close subcommands in MultiCommand (#28954)
  Build: Fix meta modules to not install as plugin in tests (#29150)
  ...