docs: update v1.18 documentation - #192

Adds TestGrpcCredentials to the existing grpc feature suite, exercising
the WithGrpcServerCredentials / WithGrpcClientCredentials surface
documented at goravel/docs#192:

  - TLS round-trip via a per-case in-process *grpc.Server with
    grpc.Creds(serverCreds), driven from the same facade that
    production code uses. Asserts the response via protobufproto.Equal
    (protojson's whitespace format has been observed to vary between
    invocations on the same source).

  - Fallback to insecure when the server entry omits ,
    matching the v1.17 backward-compat path.

  - Warning + insecure fallback when the config references an
    unregistered credentials group, captured via color.CaptureOutput.

  - Silent insecure fallback when groups are registered but the config
    key is empty, distinct from the unknown-group path above.

  - Per-key overwrite of ClientCredentials registrations, verified by
    standing up two unrelated CA/server pairs and pointing the client
    at the second one. If the first registration won, the handshake
    against the second server would fail with a cert verify error.

The running app's gRPC server is already frozen by the time the suite
runs (routes/grpc.go calls facades.Grpc().Server() to register the
UserService, which seeds the server singleton), and ServerCredentials
is silently dropped once the server has been built. The per-case
in-process *grpc.Server is the smallest change that still exercises
the real client-side resolveClientCredentials wiring through the
example's bootstrap.

A new tests/grpc/certs.go helper generates throwaway CA + localhost
server pairs (mirroring tests/telemetry/certs.go), with two
independent calls producing two unrelated trust chains so the
overwrite test can detect which registration wins.

Adds TestGrpcCredentials to the existing grpc feature suite, covering
the client side of the gRPC transport credentials surface documented at
goravel/docs#192:

  - Insecure fallback when the server entry omits `credentials`,
    matching the v1.17 backward-compat path.
  - Warning + insecure fallback when the config references an
    unregistered credentials group, captured via color.CaptureOutput.
  - Silent insecure fallback when groups are registered but the config
    key is empty, distinct from the unknown-group path above.

The server side (WithGrpcServerCredentials) is out of scope: the running
app's gRPC server is already frozen by the time the suite runs
(routes/grpc.go calls facades.Grpc().Server() to register the
UserService, which seeds the server singleton, and the framework has no
path to reset the frozen server), and the framework's app construction
helpers (foundation.NewApplication, foundation.Setup().Create,
app.Boot) either skip the gRPC provider or run the artisan/runner
pipeline that races the test. Covering the server side would require
a separate goravel/framework change to expose a server-reset path; that
is tracked separately.

The three remaining cases use the singleton's running insecure server
on 127.0.0.1:3002, so they exercise the real resolveClientCredentials
resolver rather than a stub.