feat(mongoose): re-use existing connections with request scope

[marluanespiritusanto]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
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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
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

When you try to implement a multi tenancy module, a new connection is created for each incoming request.

What is the new behavior?

Re-use existing mongoose connections when request scope is used. This makes more sense when you need to use a different connection for each request (Multi tenancy)

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[kamilmysliwiec]

Connections won't be reused within the request-scoped because the Connection provider itself is statically scoped (SINGLETON). Making this module request-scoped compatible would require a bunch of changes + we need to keep in mind that strategies on how to reuse/share/create connections vary by app strategy. Thank you for your contribution though!