feat(microservices): exposes base context on the main package

[delucca]

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: Exposes interface

What is the current behavior?

The BaseRpcContext class was not being exposed in the main package

What is the new behavior?

Now, the class is being exposed.

Does this PR introduce a breaking change?

• Yes
• No

[coveralls]

Pull Request Test Coverage Report for Build b61ea5fb-8354-4064-b4df-97d9a25958c0

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 94.108%

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Totals
Change from base Build 3a79ae39-22dd-4c90-92e4-77db246e2790:	0.001%
Covered Lines:	5782
Relevant Lines:	6144

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💛 - Coveralls

[delucca]

@kamilmysliwiec thanks! :)

I can't merge the PR (since it is only allowed to authorized users). So I'll wait for someone to merge it.