Add Connect Enterprise entry point

[gzdunek]

The first step of #17706.

e counterpart https://github.com/gravitational/teleport.e/pull/3946

A few notes:

• In the config files I use both absolute and relative paths.
  • The absolute ones are used for the source code, icons, scripts, etc, since we generally want both OSS and Enterprise use the same files (and Enterprise can override them).
  • The relative paths are used for input files/output paths - we want to build files stay local for both app versions.
• Electron version and app dependencies in externalizeDepsPlugin can no longer be resolved automatically, because they are read from the direct package.json so teleterm.e package.json doesn't have them.
  • Instead they are always read from the OSS package.json.
• The scenario when one app depends on the another doesn't work well in electron-builder. When I added @gravitational/teleterm as a dependency to @gravitational/teleterm.e the app has grown 3x times because it just copied @gravitational/teleterm to the app package. I believe it happens somwhere in app-builder, but I didn't find a way to pass excludedDependencies there.
  • Instead, I'm just ignoring '!**/node_modules/@gravitational/teleterm/**' in files. Seems to work fine, both OSS and Enterprise DMGs are now ~128.5 MB.

@ravicious you mentioned in your WIP PR this:

• teleterm.e builder-debug.yml doesn’t include '!build_resources{,/**/*}'
  How important it is? I quickly tried excluding it with!build_resources and it resulted in 3 KB size difference, so I'm not sure if I should dig there.

[ravicious]

What if teleterm.e needs to depend on deps that teleterm doesn't depend on? teleport.e already depends on extra deps, so sooner or later teleterm.e will have its own deps as well.

[ravicious]

Overall this looks okay, barring the question of teleterm.e specific deps. But I just raised a question on Slack if we even need teleterm.e in the first place.

[gzdunek]

We will move access requests to OSS and get rid of teleterm.e.