feat: Add support for running functions emulator per codebase

[taeold]

Functions emulator now only loads function codebases that are specified by the --only flag, e.g.

Given the following function configuration in firebase.json:

{
  "functions": [
    {
      "source": "js",
      "codebase": "javascript"
    },
    {
      "source": "ts",
      "codebase": "typescript",
      "predeploy": "npm --prefix \"$RESOURCE_DIR\" run build"
    }
  ],
}

You can start the emulator to only start the typescript codebase by using the --only flag like this:

$ firebase emulators:start  --only functions:typescript
i  emulators: Starting emulators: functions
i  functions: Skipping codebase javascript due to --only filter

✔  functions: Loaded functions definitions from source: helloTS.

Fixes #4878

[Copilot]

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

[Copilot]

[nitpick] There is an inconsistency: the computed default value ('default') is used for filtering, but the pushed backend still uses the original cfg.codebase (which may be undefined). Consider assigning the computed value to the backend (or clarifying the intended behavior) so that filtering and backend configuration are consistent.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 74.28571% with 9 lines in your changes missing coverage. Please review.

Project coverage is 51.23%. Comparing base (c5f77ed) to head (c3f9131).

Files with missing lines	Patch %	Lines
src/emulator/controller.ts	66.66%	7 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8422      +/-   ##
==========================================
+ Coverage   51.20%   51.23%   +0.03%     
==========================================
  Files         425      425              
  Lines       30394    30418      +24     
  Branches     6232     6238       +6     
==========================================
+ Hits        15562    15584      +22     
  Misses      13442    13442              
- Partials     1390     1392       +2     

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