feat: EnvironmentConfigBuilder mode=Token default value support

[jeacott1]

It would be very helpful if the EnvironmentConfigBuilder Token mode supported configuration of default values in a similar way to the standard java construct ${SOME_ENV_VAR: defaultValue} (note the value after the colon)

This would allow non breaking usage of tokens in existing configurations and ease gradual migration as this pattern also more closely mimics the mechanism in .net 5+ where the json content already behaves as default config without requiring templating or other intrusive mechanisms.