allow to select rmq plan when creating instance

[markusgust]

closes #36

Ensuring that we can select RMQ version on create. Couple pointers I learned while testing:

• It is not possible to pass lmq version (except test- and dev- instances) so the field should only apply to RMQ. Completion only works when selecting rmq plans. Will still allow for version to be passed, it will just be ignored by the api.
• The undocumented API endpoint returns available versions (not deprecated/beta) which I think is ok to use for completion. But it introduces makeExternalRequest.

[Copilot]

Pull request overview

Adds support for selecting a RabbitMQ broker version during instance creation, including shell completion for available versions.

Changes:

• Introduces a new --rmq-version flag on instance create and wires it into the create request payload.
• Adds shell completion that fetches and caches available RabbitMQ versions from a metadata endpoint.
• Extends the client with an external-request helper and a ListVersions() API.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
cmd/instance_create.go	Adds --rmq-version flag and passes it through to instance creation requests.
cmd/completion_helpers.go	Implements completeVersions to complete RabbitMQ versions based on selected plan.
cmd/completion_cache.go	Adds a cache TTL for version completion results.
client/regions.go	Adds ListVersions() calling a metadata endpoint to retrieve available RMQ versions.
client/instances.go	Adds an rmq_version field to create requests and includes it in form-encoded create calls.
client/client.go	Adds makeExternalRequest to support non-customer API metadata calls.

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[Copilot]

New behavior: when req.Version is set, CreateInstance adds rmq_version to the request body. The existing VCR tests for CreateInstance don’t exercise this path, so regressions could slip in (e.g., wrong form field name or missing JSON encoding when CopySettings is used). Add/update a test that creates an instance request with rmq_version set and asserts the outgoing request includes it (or record a new VCR cassette that covers this parameter).