Rebased: grpc/compiler: Respect filename suffix and extension during code generation (#7414)

* grpc/compiler: Respect filename suffix and extension during code generation

grpc compiler is not respecting filename suffix and extension passed to
flatc CLI. This causes compiler to spit out incorrect code, which then
cannot be compiled without modification.

Following patch fixes the problem.

Note, I ended up removing some code introduced #6954 ("Have grpc include
file with correct filename-suffix given to flatc") in favour of keeping
sanity of the generator code.

Signed-off-by: Aman Priyadarshi <aman.eureka@gmail.com>

* tests: Add filename-suffix and filename-ext test files

* Test 1: Filename extension changed to "hpp".
* Test 2: Filename suffix changed to "_suffix".
* Test 3: Filename extension changed to "hpp" and suffix changed to "_suffix"

Signed-off-by: Aman Priyadarshi <aman.eureka@gmail.com>