Add web app selection to app hosting onboarding

[mathu97]

Description

Adds a prompt during App Hosting onboarding to make sure user enters a Firebase web app that they would like to associate their Backend with.

Sample Commands

For interactive app selection:

• firebase apphosting:backends:create --project=<project-name>

Otherwise:

• firebase apphosting:backends:create --app="<firebase-web-app>" --project=<project-name>
• firebase apphosting:backends:create -a <firebase-web-app> --project=<project-name>

Note: Don't merge till backend appId field is supported by App Hosting backend

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 51.92308% with 25 lines in your changes are missing coverage. Please review.

Project coverage is 54.53%. Comparing base (e1aa2bf) to head (5840489). Report is 6 commits behind head on master.

Files	Patch %	Lines
src/apphosting/app.ts	57.77%	16 Missing and 3 partials :warning:
src/apphosting/index.ts	14.28%	6 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6875      +/-   ##
==========================================
- Coverage   54.79%   54.53%   -0.26%     
==========================================
  Files         359      360       +1     
  Lines       25112    25354     +242     
  Branches     5176     5249      +73     
==========================================
+ Hits        13760    13828      +68     
- Misses      10114    10283     +169     
- Partials     1238     1243       +5     

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