Allow named channels to inherit configuration from default-channel

Named channels now inherit settings from default-channel configuration.
Explicitly configured values in named channels take precedence over defaults.

Implements mergeWith() method in ChannelConfig that merges configurations
using Optional.ofNullable().orElse() pattern for null-safe value selection.

Adds ChannelInheritanceAPI tests to verify inheritance behavior.

[resolves #345]

Signed-off-by: Oleksandr Shevchenko <shevchenko.olexandr96@gmail.com>
Signed-off-by: Oleksandr Shevchenko <oleksandr.shevchenko@datarobot.com>

Author: o-shevchenko