adding region availability (#403)

## 📝 Description

**What does this PR do and why is this change necessary?**

Adding support for `regions/availability` and `regions/{region_id}/availability` endpoints.

## ✔️ How to Test

**How do I run the relevant unit/integration tests?**

`make ARGS='-run TestRegionsAvailability_List' fixtures`

regions_availability.go

@@ -0,0 +1,80 @@
+package linodego
+
+import (
+	"context"
+	"fmt"
+	"net/url"
+
+	"github.com/go-resty/resty/v2"
+)
+
+// Region represents a linode region object
+type RegionAvailability struct {
+	Region    string `json:"region"`
+	Plan      string `json:"plan"`
+	Available bool   `json:"available"`
+}
+
+// RegionsAvailabilityPagedResponse represents a linode API response for listing
+type RegionsAvailabilityPagedResponse struct {
+	*PageOptions
+	Data []RegionAvailability `json:"data"`
+}
+
+// endpoint gets the endpoint URL for Region
+func (RegionsAvailabilityPagedResponse) endpoint(_ ...any) string {
+	return "regions/availability"
+}
+
+func (resp *RegionsAvailabilityPagedResponse) castResult(r *resty.Request, e string) (int, int, error) {
+	res, err := coupleAPIErrors(r.SetResult(RegionsAvailabilityPagedResponse{}).Get(e))
+	if err != nil {
+		return 0, 0, err
+	}
+	castedRes := res.Result().(*RegionsAvailabilityPagedResponse)
+	resp.Data = append(resp.Data, castedRes.Data...)
+	return castedRes.Pages, castedRes.Results, nil
+}
+
+// ListRegionsAvailability lists Regions. This endpoint is cached by default.
+func (c *Client) ListRegionsAvailability(ctx context.Context, opts *ListOptions) ([]RegionAvailability, error) {
+	response := RegionsAvailabilityPagedResponse{}
+
+	endpoint, err := generateListCacheURL(response.endpoint(), opts)
+	if err != nil {
+		return nil, err
+	}
+
+	if result := c.getCachedResponse(endpoint); result != nil {
+		return result.([]RegionAvailability), nil
+	}
+
+	err = c.listHelper(ctx, &response, opts)
+	if err != nil {
+		return nil, err
+	}
+
+	c.addCachedResponse(endpoint, response.Data, &cacheExpiryTime)
+
+	return response.Data, nil
+}
+
+// GetRegionAvailability gets the template with the provided ID. This endpoint is cached by default.
+func (c *Client) GetRegionAvailability(ctx context.Context, regionID string) (*RegionAvailability, error) {
+	e := fmt.Sprintf("regions/%s/availability", url.PathEscape(regionID))
+
+	if result := c.getCachedResponse(e); result != nil {
+		result := result.(RegionAvailability)
+		return &result, nil
+	}
+
+	req := c.R(ctx).SetResult(&RegionAvailability{})
+	r, err := coupleAPIErrors(req.Get(e))
+	if err != nil {
+		return nil, err
+	}
+
+	c.addCachedResponse(e, r.Result(), &cacheExpiryTime)
+
+	return r.Result().(*RegionAvailability), nil
+}