Support default connection via hasCustomConnect

[sequencer]

Type of issue: Feature Request

Is your feature request related to a problem? Please describe.
Add default connection feature support to Connectable.

Describe the solution you'd like
I'm not sure should we use Bundle.Lit here, but we need to programming decide if connect a field via default or connect the field.
e.g. A full AXI bundle, parametrized not supporting AxProt or AxCache field, when bulking connect, the field at master side should be ignored, and connect a Const value defined by specification.

Describe alternatives you've considered
Exposing this feature to user maybe dangerous? In general, for those unconnected signals, users always connect to DontCare, which is more dangerous, since the behavior of DC is 0.U in compiler.

Additional context
RocketChip provides this feature via BundleMap

What is the use case for implementing this feature?

• Interconnect automation.