Move some code within src/inet files

[kpschoedel]

Problem

Code for different Inet implementations is scattered across many #ifs.

Change overview

This is a step toward #7715 Virtualize System and Inet interfaces.

• For the affected .cpp files, code is moved to a single #if section
  for each of the three implementations (LwIP, sockets, and Network
  Framework.) Some methods have internals split off into separate …Impl
  methods.

• Method implementations are reordered to match their declarations.

• Doxygen comments move to headers.

Since comparing moved code in diff format is difficult, this change
explicitly does NOT make any changes to function bodies, beyond any
necessary arguments and returns when splitting.

Testing

CI; no change to functionality.

[kpschoedel]

Note to reviewers: this PR avoids changes to moved code, since it's impractical to identify and review such changes using GitHub.

[github-actions]

Size increase report for "esp32-example-build" from 8a4eefb

File	Section	File	VM
chip-all-clusters-app.elf	.flash.text	244	244

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-all-clusters-app.elf and ./pull_artifact/chip-all-clusters-app.elf:

sections,vmsize,filesize
.debug_loc,0,2928
.debug_info,0,2440
.debug_line,0,1236
.debug_str,0,1055
.debug_frame,0,548
.strtab,0,516
.debug_ranges,0,344
.flash.text,244,244
.symtab,0,128
.debug_aranges,0,120
.debug_abbrev,0,56
.riscv.attributes,0,1
[Unmapped],0,-244

[github-actions]

Size increase report for "nrfconnect-example-build" from 8a4eefb

File	Section	File	VM
chip-shell.elf	text	68	68
chip-shell.elf	device_handles	-4	-4
chip-lock.elf	text	68	68
chip-lock.elf	device_handles	12	12

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-shell.elf and ./pull_artifact/chip-shell.elf:

sections,vmsize,filesize
.debug_info,0,1790
.debug_loc,0,1404
.debug_str,0,756
.debug_line,0,368
.strtab,0,251
.debug_frame,0,220
.debug_abbrev,0,206
.debug_ranges,0,176
.symtab,0,128
text,68,68
.debug_aranges,0,64
.shstrtab,0,-3
device_handles,-4,-4

Comparing ./master_artifact/chip-lock.elf and ./pull_artifact/chip-lock.elf:

sections,vmsize,filesize
.debug_info,0,1790
.debug_loc,0,1484
.debug_str,0,756
.debug_line,0,368
.strtab,0,251
.debug_frame,0,220
.debug_abbrev,0,206
.debug_ranges,0,176
.symtab,0,128
text,68,68
.debug_aranges,0,64
device_handles,12,12
.shstrtab,0,-3

[woody-apple]

@mrjerryjohns @saurabhst @andy31415 @Damian-Nordic @jelderton @jepenven-silabs @jmartinez-silabs ?