backport VRF support from equinix/equinix provider

[displague]

This is a backport of equinix/terraform-provider-equinix#129

Mostly derived from
https://patch-diff.githubusercontent.com/raw/equinix/terraform-provider-equinix/pull/129.patch
with some automated changes to the patch and manually cherry-picked changes from the rejects.

# vim commands applied against the patch
:%s_equinix/_metal/
:%s/equinix_//
:%s/data_source_metal/datasource_metal
:%s/_acc_test.go/_test.go
:%s/package equinix/package metal
:%s#docs/resources/metal_#docs/resources/
:%s#docs/data-sources/metal_#docs/data-sources/

Patch applied with:

patch -p1 --input=129.patch --fuzz=16

Rejects reviewed with:

find . -name \*rej  -exec less {} \;

Notably, rejects that dealt with test locations and plans were ignored for this PR.
Some helper tools for set/list conversion and sweeper test detection where copied from equinix/equinix.

[codecov-commenter]

Codecov Report

Merging #224 (8cdef96) into main (dd891eb) will increase coverage by 3.53%.
The diff coverage is 87.40%.

@@            Coverage Diff             @@
##             main     #224      +/-   ##
==========================================
+ Coverage   70.91%   74.45%   +3.53%     
==========================================
  Files          48       50       +2     
  Lines        6330     6659     +329     
==========================================
+ Hits         4489     4958     +469     
+ Misses       1643     1475     -168     
- Partials      198      226      +28     

Impacted Files	Coverage Δ
metal/datasource_metal_vlan.go	86.50% <0.00%> (-2.11%)	⬇️
metal/config.go	50.94% <30.00%> (-5.88%)	⬇️
metal/datasource_metal_reserved_ip_block.go	85.23% <84.61%> (+0.30%)	⬆️
metal/resource_metal_reserved_ip_block.go	82.67% <86.36%> (+3.41%)	⬆️
metal/resource_metal_vrf.go	86.50% <86.50%> (ø)
metal/datasource_metal_vrf.go	89.58% <89.58%> (ø)
metal/resource_metal_virtual_circuit.go	77.27% <92.85%> (+7.49%)	⬆️
metal/datasource_metal_gateway.go	100.00% <100.00%> (ø)
metal/datasource_metal_precreated_ip_block.go	73.78% <100.00%> (+0.78%)	⬆️
metal/provider.go	100.00% <100.00%> (ø)
... and 13 more

---

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