feat: migrate ecx resources to fabric v4

[ocobles]

migrated resources:

• data "equinix_ecx_l2_sellerprofile" --> data "equinix_fabric_service_profiles"
• data "equinix_ecx_port" --> data "equinix_fabric_ports"
• resource "equinix_ecx_l2_connection" --> resource "equinix_fabric_connection"

Although it might be possible to split this into smaller PRs, the primary complexity lies in the migration of the equinix_fabric_connection resource, which requires a full migration at once. Considering the complexity, I'll outline the key points below:

• The input/output variables have been kept compatible to avoid necessitating a major release. Some optional variables have been added, and two have been deprecated (speed_unit and secondary_speed_unit).
• I've moved the local variables to a separate file for better readability.
• This module simplifies the creation of fabric v4 connections through the inference of some complex fields, such as connection.type, access_point.type and access_point.link_protocol.type.
  While the functionality to support FCR and Fabric Network is already defined, I've left that code commented out with a TODO to add it in future releases.

This approach ensures minimal disruption

[thogarty]

LGTM. Also a note: FCR and Network connection types are already available and supported. You can add them at any time.

[displague]

Before including this in an 0.5.0 release, let's chase this PR with one that adds the README.md auto-updater workflow and updates the README.md (the modules in the README were not updated in this PR).

After that we can chase update the modules that depend on this module:

• https://github.com/orgs/equinix-labs/repositories?q=fabric-connection

We also discussed that another PR would introduce the TODO commented out sections. That could be done following the 0.5.0 release.