ExternalProject: Optionally integrate 'make' job server with INSTALL_COMMAND

Add an `INSTALL_JOB_SERVER_AWARE` option to `ExternalProject_Add`.
When using an explicit `INSTALL_COMMAND`, the generated commands
won't use `$(MAKE)` thus failing to connect to the outer make's
job server.  Add an option enable explicit job server integration.

This is the install step's equivalent to the build step's
`BUILD_JOB_SERVER_AWARE` option added by commit bc43398e72
(ExternalProject: Enable Make Job Server with Explicit Build Command,
2023-08-09, v3.28.0-rc1~217^2).  It is useful when the external
project's installation is driven by its build system.  Note that with
Makefile generators, our default install command does use `$(MAKE)` to
connect to the outer make's job server.

Issue: #26398

Modules/ExternalProject/shared_internal_commands.cmake

@@ -1934,6 +1934,7 @@ macro(_ep_get_add_keywords out_var)
     #
     INSTALL_COMMAND
     INSTALL_BYPRODUCTS
+    INSTALL_JOB_SERVER_AWARE
     #
     # Test step options
     #