Fix pagination to fetch all release assets

[IvanChalukov]

Closes: #139

Fix pagination to fetch all release assets and add coverage tests.

Related:
#133
#134

Notes to reviewer

Eliminates the premature break to correctly iterate over all pages of assets using res.NextPage,
resolving the previous limitation where only the first 100 assets were retrieved.

I've tested it with custom resource based on github-release-resource and following pipeline:

resource_types:
- name: custom-resource
  type: registry-image
  source:
    repository: ichalukov/custom-resource
    tag: latest

resources:
- name: openjdk-release
  type: custom-resource
  source:
    owner: bell-sw
    repository: Liberica
    access_token: ((github_token))
    semver_constraint: "~23"

jobs:
  - name: check-openjdk-release
    plan:
      - get: openjdk-release
        params:
          globs:
            - "bellsoft-jre*-linux-amd64.tar.gz"
      - task: list-assets
        config:
          platform: linux
          image_resource:
            type: registry-image
            source:
              repository: busybox
          inputs:
            - name: openjdk-release
          run:
            path: sh
            args:
              - -c
              - |
                echo "Listing contents of openjdk-release directory:"
                ls -al openjdk-release || echo "(no files downloaded)"