Add repository ruleset management functions

[MariusStorhaug]

Rulesets control how people can interact with selected branches and tags in a repository.

https://docs.github.com/en/rest/repos/rules?apiVersion=2022-11-28

[MariusStorhaug]

Flagging a sibling scope for this area, driven by MSXOrg/Custo (PR #1), which maintains rulesets centrally as an enterprise control:

• Repository rulesets — …/repos/{owner}/{repo}/rulesets (this issue).
• Organization rulesets — …/orgs/{org}/rulesets.
• Enterprise rulesets — …/enterprises/{enterprise}/rulesets.

The three share a payload shape but differ by scope. Worth deciding whether they live under one set of *-GitHubRuleset cmdlets parameterized by scope, or per-scope cmdlets (*-GitHubRepositoryRuleset, *-GitHubOrganizationRuleset, *-GitHubEnterpriseRuleset).

Auth note: enterprise rulesets require enterprise-admin authorization. GitHub Apps generally cannot manage enterprise rulesets, so a PAT with admin:enterprise is needed there — the cmdlet should accept whichever -Context token is supplied and error clearly when the scope is insufficient.

Note this is distinct from Get-GitHubRepositoryRuleSuite (rule evaluation results) — this issue is about managing ruleset definitions.