[Rust] Add ENUM_VALUES generation for bit_flags enums

[wentywenty]

This PR adds the generation of the ENUM_VALUES constant array for enums marked with the bit_flags attribute in Rust.

Motivation: Currently, standard enums generate an ENUM_VALUES array, but bit_flags enums do not. This makes it difficult to iterate over all defined base flag values (e.g., for UI lists, serialization helpers, or debugging).

Changes: Modified src/idl_gen_rust.cpp to generate an impl block containing ENUM_VALUES for bitflags, mirroring the behavior of standard enums.

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[wentywenty]

@aardappel Hello, could you spare some time to review this PR? Thank you very much!

[aardappel]

what's a typical use case for this?

[jtdavis777]

would love to see a new test that showcases this feature <3

[wentywenty]

@jtdavis777 That's a great idea! I will add a test case to demonstrate this feature.

[jtdavis777]

hey @wentywenty just checking in on that test case :)

[fawdlstty]

@wentywenty Hello. do u need some help for this?