feat: export SubscriptionClient class

[codeflyer]

in this PR:

• Export the SubscriptionClient
• update the type definitions
• add test to the types exported

[mcollina]

I would rather export it in a separate module instead. Wdyt?

[codeflyer]

I would rather export it in a separate module instead. Wdyt?

You mean export just the Subscription Client? Or all the subscription feature?

In the first case we can build a module like @mercurius/subscription-tools and use them directly in the mercurius part that manage the subscription.

I wont extract instead the feature completely.

Wdyt?

[mcollina]

I would extract only the SubscriptionClient.

[marco-ippolito]

I would extract only the SubscriptionClient.

this is being done here mercurius-js/mercurius-subscription-client#3

[marco-ippolito]

@mcollina @codeflyer we can close this has has been resolved by #938