Simplify and unify the constructor and option API

[meling]

Overview

Simplify and unify the constructor and option API of Gorums by removing ambiguity, enforcing better type safety, and hiding internal complexities (like Manager).

The changes align Gorums with standard gRPC naming conventions and significantly reduce the public surface area that users need to learn.

Motivation

• Manager is an internal implementation detail that should not be part of the public API.
• ManagerOption is an awkward name; DialOption aligns with gRPC conventions and mirrors ServerOption / CallOption.
• The two-step NewManager + NewConfiguration pattern is redundant; a single NewConfig suffices.
• (*Server).NewConfig duplicates gorums.NewConfig(..., gorums.WithServer(srv)).
• Configuration.Manager() exposes internal state unnecessarily.

Sub-issues

• refactor: rename ManagerOption to DialOption #295 Rename ManagerOption to DialOption (closes chore: rename ManagerOption to ClientOption? #222)
• refactor: unexport Manager as outboundManager #296 Unexport Manager as outboundManager
• feat: export WithServer dial option; remove Server.NewConfig #297 Export WithServer dial option; remove (*Server).NewConfig
• refactor: simplify NewConfig signature; remove NewConfiguration and Manager alias #298 Simplify NewConfig signature; remove NewConfiguration and Manager alias
• refactor: update all callers to new NewConfig signature #299 Update all callers to new NewConfig signature
• chore: regenerate protobuf and gorums generated files #300 Regenerate protobuf and gorums generated files

Verification

go test -v -timeout=15s ./...
go test -tags=integration ./...

All tests must pass.