C++: allow iterating over unique enum values

[pwrobeldev]

---------- Motivation ----------
There were a few requests in the past to provide the means for
users of the generated code to iterate over enumeration values.

---------- Solution ----------
Introduced a new cpp-specific attribute in LIME language called 'EnumValues'.
When LIME enumeration is annotated with it, then C++ generator creates additional
helper function that returns the array of unique enumerators. It is achieved via the new
mustache templates. The change implements smoke and functional tests to confirm
that the new functionality works as expected.

For the following input:

@Cpp(EnumValues)
enum EnumWithAccessibleValues {
    FOO,
    BAR,
    BAZ,
    FOO_ALIAS = FOO
}

The additional function is generated:

std::array<EnumWithAccessibleValues, 3>
EnumWithAccessibleValues_enumerators();

[pwrobeldev]

The failure for Swift on MacOS seems to be related to environment issues related to the used runner / action to setup JDK. I will investigate it and fix in a separate MR.

In meantime, I executed all functional tests on my local MacOS with the current branch -- there are no problems.

1: Test Suite 'testfunctional.xctest' passed at 2026-05-25 19:12:27.586.
1: 	 Executed 501 tests, with 0 failures (0 unexpected) in 0.063 (0.072) seconds
1: Test Suite 'All tests' passed at 2026-05-25 19:12:27.586.
1: 	 Executed 501 tests, with 0 failures (0 unexpected) in 0.063 (0.076) seconds
1/1 Test #1: XCTest.functional ................   Passed    1.42 sec

[pwrobeldev]

The changes have been merged. The environment issue for Swift on MacOS job will be investigated and fixed in a separate MR.