Remove the need to specify IDL env var location

[ivmarkov]

This PR removes the need for the RS_MATTER_IDL_DIR environment variable that points to the location of the Matter Clusters' IDL file.

The inconvenience of this environment variable should not be underestimated:

• While the rs-matter project does have a .cargo/config.toml file listing the env var, this is only helping the Continuous integration build of the rs-matter library itself
• Yet, when listing a dependency to rs-matter, the user still has to declare its own version of the env var which ideally must point to... the same IDL file which is checked-in in the rs-matter GIT repo. So in a way, the user must have the rs-matter repo checked out somewhere, just because of the IDL file; UPDATE: moreover, the checked out rs-matter GIT repo should correspond exactly to the one published on crates.io or else, it might mis-compile itself, if we - ourselves - use the IDL file to generate our own clusters - which is already the case albeit for demoing purposes - with the on_off cluster.

The PR completely bypasses all of this by

• Moving the rs-matter/idl folder as a sub-folder of rs-matter-macros-impl/src
• Using include_str! to include the content of the IDL file - at build time - in the macro generating code

[ivmarkov]

@andy31415 @kedars ^^^

[ivmarkov]

@ivmarkov should we add a separate issue to support custom clusters?

Right now our macros will include all CSA-spec defined clusters (and a unit-testing one) however custom app clusters would need a different IDL.

Low priority since anyone can use the example in rs-matter-macros to create their own macro.

Yes, let's at least have an issue for that.