fix: add credential runtime hint for graalvm compilation

[zhumin8]

fixes #2569

As reported in #2569, Graalvm is not detecting Credentials class used in GcpProperties, even though Credentials has been marked by @NestedConfigurationProperty.
What is affected?

• spring-cloud-gcp-core external configurations for spring.cloud.gcp.credentials.encoded-key and spring.cloud.gcp.credentials.location
• module specific configs for credentials. e.g. spring.cloud.gcp.firestore.credentials.encoded-key

What is not affected?
Setting GOOGLE_APPLICATION_CREDENTIALS goes through ADC and does not involve spring.cloud.gcp.credentials.encoded-key property, thus is not affected.

Changes in this pr:

• Added runtime hint for Credentials on GcpProperties to resolve this. This change also fixes for non-core module's autoconfiguration because each of them depends on GcpContextAutoConfiguration from core. e.g. GcpFirestoreAutoConfiguration
• Added test to starter firestore sample:
  • test: FirestoreSampleAppNativeIntegrationTests.java, this test only runs when "it.firestore.native=true", and does not run the CommandRunner in sample app.
  • test setup:
    • pass encoded ci sa key to native test config for firestore samples.
    • enable this new test for native test config

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud