[improve][fn] Add Go support for pulsar function secrets

[DavidRayner]

Fixes #8425

Motivation

Currently the Java and Python pulsar functions can access secrets that are set when functions are installed, but not pulsar functions written in Go.

Modifications

• The pulsar function runtime code has been updated so that secretsProviderClassName and secretsProviderConfig are passed to Go pulsar functions.
• The default secretsProviderClassName for Go functions has been set to ClearTextSecretsProvider
• Go pulsar function start-up has been updated to setup the secrets provider.
• A new Go method GetSecretValue has been added to the FunctionContext type so that a pulsar function can retrieve secrets.

Verifying this change

• [ ] Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• A pulsar function runtime unit test has been updated to ensure that secretsProviderClassName and secretsProviderConfig are passed to the Go function.
• New Go unit tests have been written to test the GetValue method for the two Go SecretsProvider implementations

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• [ ] Dependencies (add or upgrade a dependency)
• [x] The public API

A new Go method GetSecretValue has been added to the FunctionContext type.

• [ ] The schema
• [x] The default values of configurations

The default secretsProviderClassName for Go functions has been set to ClearTextSecretsProvider

• [ ] The threading model
• [ ] The binary protocol
• [ ] The REST endpoints
• [ ] The admin CLI options
• [ ] The metrics
• [ ] Anything that affects deployment

Documentation

• [ ] doc
• [x] doc-required
• [ ] doc-not-needed
• [ ] doc-complete

Pulsar-site changes here: https://github.com/apache/pulsar-site/pull/903

Matching PR in forked repository

PR in forked repository: https://github.com/DavidRayner/pulsar/pull/3