Add xxx:identifier metadata to remap identifiers

[bernardnormier]

We want to add xxx:identifier metadata (cs:identifier, cpp:identifier, java:identifier) that allows users to remap C#/C++/Java etc. to the specified identifier.

For example:

interface Greeter 
{
    ["cs:identifier:Greet"] // changes the case in C#
    string greet(string name);
}

See also:
https://docs.icerpc.dev/slice2/language-guide/attributes#cs::identifier-attribute

Then once implemented, we want to remove various escaping in the slice compilers. If the generated code does not compile due to the Slice identifier choice of the user, the user should use xxx:identifier to pick the replacement identifier of their choice.