Convert the groups and scenes clusters from C to C++.

[bzbarsky-apple]

Problem

We likely want to create a C++ API for #2168 and these clusters need to read from buffers.

Summary of Changes

Rename files from .c to .cpp

[github-actions]

Size increase report for "esp32-example-build"

File	Section	File	VM
chip-wifi-echo.elf	.dram0.bss	0	-8

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-wifi-echo.elf and ./pull_artifact/chip-wifi-echo.elf:

sections,vmsize,filesize
.debug_info,0,21319
.debug_line,0,4592
.debug_abbrev,0,2325
.debug_str,0,432
.strtab,0,204
.debug_loc,0,76
.debug_ranges,0,48
.dram0.bss,-8,0

[bzbarsky-apple]

@saurabhst @BroderickCarlin