[Rust] harden against name collisions while generate cleaner rust code, fix #20337

[xMAC94x]

This change adjust the generated rust code to be less prone to name collisions, by actually escaping the user-data instead of having a local_var_ prefix (see #20337 for details).
It uses the vendor extensions, so specific names can also be overwritten (if needed).

It also gets rid of the naming problems in a whole for grouped parameters because it gets rid of the "unbox" in the template.

The change will modify code generated with openapi-generator but should not interact with the generated interfaces, thus I think its no breaking change.

No extra tests where added and this change should validate if CI passes the integration tests.

[RUST] @frol @farcaller @richardwhiuk @paladinzh @jacob-pro
@wing328

Note: After this PR is merged, future PRs should use {{{vendorExtensions.x-rust-param-identifier}}} within the api.mustache file if they want to mention the old {{{paramName}}}

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[imaami]

VENDOR_EXTENTION_PARAM_IDENTIFIER seems like a typo, shouldn't it be VENDOR_EXTENSION_PARAM_IDENTIFIER?

[wing328]

can we rename in_function_identifier to inFunctionIdentifier instead to conform to java variable naming convention?