[C++] Support configuring optional scope field for OAuth2 authenticat…

…ion (#12305)

It's a C++ client catchup for #11931.

- Add a `scope_` field to `ClientCredentialFlow` and load it from `ParamMap` object whose key is `scope`.
- Refactor `ClientCredentialFlow` to simplify code and make it testable:
  - Use only one constructor instead of two overloaded constructors that might look confused
  - Add a `generateJsonBody` public method for generating JSON body for post fields in `authenticate` so that it can be tested.
  - Add a `KeyFile` class like what Java client does to load client id and client secret from `ParamMap` or file.

- [x] Make sure that the change passes the CI checks.

This change added test `AuthPluginTest.testOauth2RequestBody` for the cases that scope exists or doesn't exist.

(cherry picked from commit 44dcc04)

pulsar-client-cpp/lib/auth/AuthOauth2.cc

@@ -119,6 +119,14 @@ bool Oauth2CachedToken::isExpired() { return expiresAt_ < currentTimeMillis(); }
 Oauth2Flow::Oauth2Flow() {}
 Oauth2Flow::~Oauth2Flow() {}
 
+// ClientCredentialFlow
+static std::string readFromFile(const std::string& credentialsFilePath) {
+    std::ifstream input(credentialsFilePath);
+    std::stringstream buffer;
+    buffer << input.rdbuf();
+    return buffer.str();
+}
+
 KeyFile KeyFile::fromParamMap(ParamMap& params) {
     const auto it = params.find("private_key");
     if (it != params.cend()) {