GitHub Connector: support GitHub Apps (instead of PATs)

[MaxTru]

Is your feature request related to a problem? Please describe.

As per documentation (https://docs.camunda.io/docs/components/connectors/out-of-the-box-connectors/github/#prerequisites), the GitHub connector currently works with PATs.

Describe the solution you'd like

In addition to PATs, it would be great if a GitHub app could perform actions orchestrated by Camunda. In order to support this, the GitHub Connector needs to support the JWT flow to generate App Installation Access Tokens.

Describe alternatives you've considered

While PATs work, they have the downside, that they are always connectors to a user, which is not appropriate given that Camunda Processes work without user interaction.

Additional context

n/a

[MaxTru]

In analogy to generate token action: https://github.com/camunda/engineering-leadership-team/blob/main/.github/workflows/AUTO_ASSIGN_ISSUE.yml

[npepinpe]

No pressure or anything, but this is a recurring pain point for the release process and other automated processes around GitHub (e.g. external PR checker). I was going to do this at some point, but if there's a chance you will do it... 😄

[MaxTru]

@sbuettner maybe we could also mark this issue as pr welcome?

[sbuettner]

@MaxTru Added the help wanted label as announced by @xomiamoore
Context: https://camunda.slack.com/archives/G01EAM4M0HK/p1731066726414569