feat: Support IPv6 parameters when creating / updating interconnection virtual circuits

[RaptorGandalf]

This PR adds support for the newly available IPv6 parameters when creating / updating interconnection virtual circuits. To allow this feature to also be used with shared interconnections, some changes to the create logic of the interconnection virtual circuit resource were made.

With shared interconnections, the customer does not create virtual circuits, but rather they are created for them by the metal API. Attempting to create a virtual circuit on a shared connection will result in an error from the Metal API.

To get around this, an optional virtual_circuit_id parameter has been added to the virtual circuit resource. If this value is present, the terraform create function will issue an update call to the metal API for the existing specified virtual circuit, rather than trying to create a new one.

Additionally I have added support for the already existing IPv4 parameters in the update function which seemed to be missing.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 64.73988% with 61 lines in your changes missing coverage. Please review.

Project coverage is 53.71%. Comparing base (503d9e1) to head (07756d8).
Report is 86 commits behind head on main.

Files	Patch %	Lines
...ternal/resources/metal/virtual_circuit/resource.go	60.89%	50 Missing and 11 partials ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##             main     #717       +/-   ##
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+ Coverage   37.91%   53.71%   +15.79%     
===========================================
  Files         120      151       +31     
  Lines       19476    20723     +1247     
===========================================
+ Hits         7384    11131     +3747     
+ Misses      11884     9138     -2746     
- Partials      208      454      +246     

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[ctreatma]

The code changes look good; just needs at least one test to cover the IPv6 functionality.