Add support to custom ids.

[mshima]

This PR adds support to custom ids:

entity Foo {
  id UUID
}

and

entity Foo {
  @Id customId Long
}

Relationships are not complete yet.
Angular and CI changes split into #13257 and #13204

Related to #7112

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[pascalgrimaud]

Which type can we use ? UUID ? Long ?

[mshima]

Which type can we use ? UUID ? Long ?

Both with auto generation.
Others with manual generation, but I didn't try.

entity Foo {
  @AutoGenerate(false) @Id String customId
}

[mraible]

It would be cool if we could generate UUIDs instead of Long for IDs. My 21-Points Health app has been in production for over 5 years and it has issues with Long. I created this PR to fix it, but I'm hesitant to merge it because it's not a standard JHipster feature and it'll be difficult to build the app again for JHipster 7.

[mshima]

Merging this later today if there is not objections.