feat(footer): add custom footer to show metrics

[ghiscoding]

• this footer is typically only available when user is not using a Backend Service since they show the same kind of info

TODO

• full implementation
• add Jest unit tests
• add Cypress E2E tests
• create Wiki

[codecov]

Codecov Report

Merging #284 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #284   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files         153    154    +1     
  Lines        8743   8784   +41     
  Branches     2893   2810   -83     
=====================================
+ Hits         8743   8784   +41

Impacted Files	Coverage Δ
src/aurelia-slickgrid/global-grid-options.ts	100% <ø> (ø)	⬆️
src/aurelia-slickgrid/constants.ts	100% <ø> (ø)	⬆️
src/aurelia-slickgrid/models/index.ts	100% <100%> (ø)	⬆️
src/aurelia-slickgrid/index.ts	100% <100%> (ø)	⬆️
src/aurelia-slickgrid/services/resizer.service.ts	100% <100%> (ø)	⬆️
...urelia-slickgrid/value-converters/asgDateFormat.ts	100% <100%> (ø)
...lia-slickgrid/custom-elements/aurelia-slickgrid.ts	100% <100%> (ø)	⬆️

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