Use bool const type for rpc type

[smolijar]

Uses const types for requestStream and responseStream, either true
or false types instead of generic boolean TS types allowing advanced
call type inference from the service definition.

Higher abstraction framework libraries, such as
https://github.com/malijs/mali could leverage advanced type inference to
hint/check specific type handlers if this information would be part of
the type rpc service type definition.

At this point, the only way to determine thy type of RPC is using
I{{{serviceName}}}Server interface, which must be linked manually to
I{{{serviceName}}}Service methods. For registring handlers at runtime,
only e.g. GreeterService and corresponding implementation is required,
manually adding IGreeterServer is possible, but troublesome. This
would not be required if rpc type could be inferred using the const
booleans to define the type.

This change does is not a breaking change and the const boolean types
are working correctly since the oldest available version on TS
playground 2.4.1. See playground link in the trailers.

There are no obvoius disadvantages to using const booleans and change
could potentially give node grpc frameworks more power for sophisticated
type inference.

There are no tests in the project, but so at least I tried it manually
with npm link on a custom project and it behaves as expected.

See: https://www.typescriptlang.org/play/?ts=2.4.1#code/MYewdgzgLgBAhsYBTADlAKgJwK5JgXhgAoA3OAGwC4YockBKAgPhjPIChRJYFk0AxChDyFSFagDMhDZqwrt2vVBjpEp5YfUWJlgjUjXStStFlxFauYzoHSLdekA

[agreatfool]

Thanks for the pr. Let me have a look.

[agreatfool]

Sorry for the late reply. Version 4.1.0 has been published.