refactor(): interface improvement for discovery service

[Tony133]

PR Checklist

Please check if your PR fulfills the following requirements:

• 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)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• 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?

Issue Number: N/A

What is the new behavior?

Proposal:

• Moved the DiscoveryOptions interface to a separate file, now inside the interfaces folder.
• Created index.ts file inside interfaces folder.
• Update the DiscoveryService

Does this PR introduce a breaking change?

• Yes
• No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 3d183d94-d740-4510-b47c-24ec5156a3e2

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.409%

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Totals
Change from base Build 43c8609d-7c9b-40c5-85e1-094c9c517952:	0.0%
Covered Lines:	6207
Relevant Lines:	6645

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💛 - Coveralls

[kamilmysliwiec]

This introduces a breaking change

[Tony133]

I thought so 😄, in fact I have others to propose similar to this one, but I waited to do it in PR, to see what you thought. Maybe I could group them in this PR? 🤔 I just have to rename the title the PR

But for the next major version of NestJS, this could be ok? right?

[kamilmysliwiec]

We should only introduce breaking changes when it's absolutely necessary.