Add config default for setting service provider subject claim value

[ashera96]

Purpose

• Resolves: https://github.com/wso2/api-manager/issues/2932

This PR adds the default value for service_provider.use_username_as_sub_claim. With this default behaviour, on SP creation for application key generation, the default subject claim will be userID. If set to true using deployment.toml, the username will be used as the subject claim.

Related PRs

• https://github.com/wso2/carbon-identity-framework/pull/5746
• https://github.com/wso2-extensions/identity-inbound-auth-oauth/pull/2495
• https://github.com/wso2/carbon-identity-framework/pull/5745
• https://github.com/wso2-extensions/identity-inbound-auth-oauth/pull/2492