Set up Branch-specific investment cost inputs for Transmission Lines to Switch

# :rocket:

- [ ] Is your feature request essential for your project?

### Describe the workflow you want to enable
Currently, the transmission line investment cost input to Switch is a single value for the entire country.
Big picture: we want to develop a workflow that can modify any or every investment cost value for each transmission line around the country.
This is step 1 of that process: allowing for an override process while also ensuring that each transmission line has an investment cost value.

### Describe your proposed implementation
Unlike issue #98, this feature request appears to be (a little) more involved. There currently is no function like build_gen_build_costs for transmission lines and thus likely needs to be added.

Similar to Issue #98, we also likely want an optional parameter input to grid_to_switch.py that could be a Series, Dictionary, or DataFrame to set trans_capital_cost_per_mw_km for each branch.
This will need an additional column in the branch data sent to Switch

This parameter has
Index branch_id
If it is a single column, it is assumed to apply to all investment years.
If it has multiple columns, then each column is assumed to map to a particular investment year

### Describe alternatives you've considered, if relevant
None at this time.

### Additional context
There is an open question of how to handle hypothetical branches that don't correspond to original branched: similar to issue #69 for plants