feat(chat): add support for custom message template

[andreipadolin]

Please read and mark the following check list before creating a pull request:

• I read and followed the CONTRIBUTING.md guide.
• I read and followed the New Feature Checklist guide.

Short description of what this resolves:

Fixes #2134. Fixes #2279.
Update for #2413

BREAKING CHANGE:
NbChatMessageComponent constructor now has a second parameter (NbCustomMessageService).

[codecov]

Codecov Report

Merging #2750 (91bcfbf) into master (002adc2) will decrease coverage by 6.18%.
The diff coverage is 75.94%.

❗ Current head 91bcfbf differs from pull request most recent head f082f2d. Consider uploading reports for the commit f082f2d to get more accurate results

@@            Coverage Diff             @@
##           master    #2750      +/-   ##
==========================================
- Coverage   84.57%   78.39%   -6.19%     
==========================================
  Files         235      265      +30     
  Lines        8059     8265     +206     
  Branches      710      899     +189     
==========================================
- Hits         6816     6479     -337     
- Misses       1023     1503     +480     
- Partials      220      283      +63     

Impacted Files	Coverage Δ
src/framework/auth/components/auth.component.ts	28.57% <0.00%> (-15.88%)	⬇️
...framework/auth/strategies/auth-strategy-options.ts	100.00% <ø> (ø)
.../auth/strategies/oauth2/oauth2-strategy.options.ts	100.00% <ø> (ø)
...ramework/auth/strategies/oauth2/oauth2-strategy.ts	83.57% <ø> (-1.10%)	⬇️
...work/auth/strategies/password/password-strategy.ts	97.59% <ø> (-0.24%)	⬇️
...ponents/accordion/accordion-item-body.component.ts	46.15% <0.00%> (-12.67%)	⬇️
...nents/accordion/accordion-item-header.component.ts	40.00% <0.00%> (-8.28%)	⬇️
...e/components/accordion/accordion-item.component.ts	28.57% <0.00%> (-9.73%)	⬇️
.../theme/components/accordion/accordion.component.ts	41.66% <ø> (-11.67%)	⬇️
...eme/components/calendar-kit/calendar-kit.module.ts	100.00% <ø> (ø)
... and 325 more

[yggg]

We also need to update docs to tell users about custom message support. You'll find docs on the chat in the chat.component.ts file in the comment above the class declaration. Please, add several sentences on how to provide a custom message template and then reference your example via @stacked-example(Custom message, chat/chat-custom-message.component).
To start the docs app run npm run docs:serve. If you update text in the documentation comment in the chat component file you need to run npm run docs:parse to see the change in the docs app (no need to restart docs app).