feat(forRoot): Allow connection name as entry param

[LeoAnesi]

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PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
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Issue Number: #66

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

• This PR is dependent of PR 1029 that fixes eslint
• It works if you specify in every entity what connection you are using, but does not allow to use one named connection as you would do with an unnamed one, which is what I would need for example to change the database used depending if I'm running tests or dev server. I'm not sure it closes the issue
• If you have any idea to improve this / to make what I said above work I would be happy to take them into account
• As I'm not certain it brings value to this repo I'm waiting for a validation to write the corresponding doc

[LeoAnesi]

This implies a lot of changes in the code that are not easy and the doc already answers this issue with this suggestion :

TypeOrmModule.forRootAsync({
  useFactory: async () =>
    Object.assign(await getConnectionOptions(), {
      autoLoadEntities: true,
    }),
});

In that case we just have to give a name to getConnectionOptions and it will work