Add Deploy Keys to the credential inventory
### Problem to solve
Compliance-minded organizations require insight into the access individuals have to their systems. We're launching an [inventory of PAT and SSH credentials](https://gitlab.com/gitlab-org/gitlab/issues/36742) in %"12.6", which begins to provide this insight to customers. Missing still is `Deploy Keys` so that customers can have a more holistic view of access to their GitLab environment.
### Intended users
* [Sidney (Systems Administrator)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sidney-systems-administrator)
Personas are described at https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/ -->
### Further details
This is an iteration to add `Deploy Keys` to an existing credential inventory accessible by `Administrators`.
### Proposal
Add `Deploy Keys` to the [credential inventory](https://gitlab.com/gitlab-org/gitlab/issues/36742).

|Title|Fingerprint|Read-only access|Read-write access|Created|(Actions)|
|---|---|---|---|---|---|
|Admin deploy key|d2:e8:be:cf:fd:a7:b4:fd:a7:b4 |2 projects|1 project|20201-01-01||
<details><summary>Empty state</summary>
|Preview|
|:---:|
|[illustrations_empty-state_empty-deploy-keys-lg.svg](/uploads/ca3e11cd29fbaed945028a347156914c/illustrations_empty-state_empty-deploy-keys-lg.svg)](https://gitlab.com/gitlab-org/gitlab-svgs/blob/main/illustrations/empty-state/empty-deploy-keys-lg.svg) <br> <br> **Deploy keys allow read-only or read-write (if enabled) access to your repository** <br> <br> Deploy keys can be used for CI, staging or production servers. You can create a deploy key or add an existing one. <br> <br> |
</details>
[docs](https://docs.gitlab.com/ee/user/project/deploy_keys/#deploy-keys)
### Permissions and Security
Only `Administrators` can view this information within the `Admin Panel`
### Designs
| Table | Empty state |
| -- | -- |
|  |  |
### Implementation plan
Integration plan
| Expertise | Issue | Weight |
| --------- | ----- | ------ |
| ~"feature flag" | [Create feature flag](https://gitlab.com/gitlab-org/gitlab/-/issues/300760) | :one: |
| ~backend | [Backend implementation](https://gitlab.com/gitlab-org/gitlab/-/issues/300762) | :three: |
| ~frontend | [Remove On suffixes](https://gitlab.com/gitlab-org/gitlab/-/issues/300763) | :one: |
| ~frontend | [Frontend implementation](https://gitlab.com/gitlab-org/gitlab/-/issues/300764) | :three: |
| ~frontend | [Add all empty states](https://gitlab.com/gitlab-org/gitlab/-/issues/351883) | :two: |
epic