Include a can_write_custom_eui64 stack-specific key in the backup

[puddly]

Will be used to show a checkbox for EZSP radios in the ZHA backup/restore dialog, to allow writing the radio EUI64 address.

[codecov-commenter]

Codecov Report

Merging #469 (940cfbf) into dev (d73b2f0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev     #469   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files          50       50           
  Lines        4187     4194    +7     
=======================================
+ Hits         4184     4191    +7     
  Misses          3        3           

Impacted Files	Coverage Δ
bellows/ezsp/__init__.py	98.55% <100.00%> (+0.02%)	⬆️
bellows/zigbee/application.py	100.00% <100.00%> (ø)

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