feat: `GET /repos/{owner}/{repo}/code-scanning/alerts/{alert_number}/…

…instances`, `GET /repos/{owner}/{repo}/code-scanning/analyses/{analysis_id}`, `GET /repos/{owner}/{repo}/code-scanning/sarifs/{sarif_id}`. Improved parameter types and responses for code scanning endpoints (#52)

BREAKING CHANGE: `code-scanning-alert-instances` schema removed.

The instances response field is deprecated and will, in future, not be included in the response of code scanning alerts endpoints. The same information can now be retrieved via a GET request to the URL specified by `instances_url`.

BREAKING CHANGE: schema `code-scanning-alert-ref` renamed to `code-scanning-ref`

BREAKING CHANGE: schema `code-scanning-alert-rule` renamed to `code-scanning-alert-rule-summary`

BREAKING CHANGE: schema `code-scanning-alert-code-scanning-alert-items` renamed to `code-scanning-alert-items`

BREAKING CHANGE: schema `code-scanning-alert-code-scanning-alert` renamed to `code-scanning-alert-rule`

BREAKING CHANGE: schema `code-scanning-analysis-ref` renamed to `code-scanning-ref`

BREAKING CHANGE: schema `code-scanning-analysis-code-scanning-analysis` renamed to `code-scanning-analysis`