feat: add optional metal_base (EQUINIX_API_METAL_BASE) provider config

[displague]

Adds support for alternate Equinix Metal Base Path.
This is used in Prism spec validation: https://gist.github.com/displague/27363182851f0eab4b599401f83f4b1e.

There may not be other real-world test scenarios today outside of the use-case described. If there were a /metal/v1{suffix}, this could be helpful.

An alternative to this base path variable would be one that replaces the full Metal URL. This would provide ways to test against the legacy api.packet.net path structure (and other environments) for example.

[ctreatma]

I don't think we should do this. For the Prism case, that seems to be an issue with Prism (it doesn't respect the base path that is baked in to servers.0.url) and could possibly be worked around with something like this or by making a contribution to Prism that would have that proxy respect any base path in the specified server URL. For the api.packet.net case, IMO that should be difficult to configure; our customers should use (and be able to rely on) our standard api.equinix.com domains.

[displague]

Closing based on @ctreatma's recommendations for alternate approaches and because we haven't seen the Spotlight integration manifest since this PR was created.