Add ChipMdnsShutdown()

[kpschoedel]

Problem

Issue #8001 MdnsAvahi destructor segfault; require separate shutdown:
MdnsAvahi can cause a crash on exit due to a static instance running
code in its destructor after dependent components have shut down.

Change overview

• Add ChipMdnsShutdown(). This is a narrow fix, which does not attempt
  to address other potential problems with singleton mDNS state.
• Fix TestMdns to shut down cleanly.

Testing

• TestMdns and CI.

[github-actions]

Size increase report for "esp32-example-build" from 0bdeccb

File	Section	File	VM
chip-lock-app.elf	.flash.text	-68	-68
chip-bridge-app.elf	.flash.text	-48	-48

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

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

sections,vmsize,filesize
[Unmapped],0,68
.flash.text,-68,-68

Comparing ./master_artifact/chip-bridge-app.elf and ./pull_artifact/chip-bridge-app.elf:

sections,vmsize,filesize
[Unmapped],0,48
.flash.text,-48,-48

Comparing ./master_artifact/chip-ipv6only-app.elf and ./pull_artifact/chip-ipv6only-app.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-pigweed-app.elf and ./pull_artifact/chip-pigweed-app.elf:

sections,vmsize,filesize

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

sections,vmsize,filesize

Comparing ./master_artifact/chip-temperature-measurement-app.elf and ./pull_artifact/chip-temperature-measurement-app.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-persistent-storage.elf and ./pull_artifact/chip-persistent-storage.elf:

sections,vmsize,filesize

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

sections,vmsize,filesize