Use `direct_asset_path` instead of `filepath` when calling Releases or Release Links APIs
### Overview
The [Release](https://docs.gitlab.com/ee/api/releases) and [Release Links](https://docs.gitlab.com/ee/api/releases/links.html) APIs accept a `filepath` parameter which represents a [permanent link to an asset](https://docs.gitlab.com/ee/user/project/releases/index.html#permanent-links-to-release-assets).
This parameter will be [deprecated and removed](https://gitlab.com/groups/gitlab-org/-/epics/9661) in %"17.0", and an alias (e.g. `direct_asset_path`) will be available to use instead.
That alias will be available starting in %"15.9" (check https://gitlab.com/gitlab-org/gitlab/-/issues/273757 to keep track of progress).
We want to ensure any references to the old parameter in the CLI is replaced with `direct_asset_path` moving forward.
### Proposal
- Ensure [`Link` struct](https://gitlab.com/gitlab-org/release-cli/-/blob/fbf151362d1306af3a72c9fdf590a2d7201d05d9/internal/gitlab/release.go#L30-38) is used only for responses. For requests, we have to use `DirectAssetPath` instead of `Filepath`.
- Update the [corresponding test](https://gitlab.com/gitlab-org/release-cli/-/blob/fbf151362d1306af3a72c9fdf590a2d7201d05d9/internal/gitlab/release_test.go#L38) to reflect the change above (i.e. use `DirectAssetPath`).
issue