feat: github apps support

[klopfdreh]

Fix for: #3188

Things need to be done before the feature is ready:

• The dependencies have to be managed in a better way and also may exchanged with alt. integrations (jsonwebtoken / bouncycastle)
• The Properties could be renamed and relocated to a Properties class
• Proxy-Settings for RestTemplate setup should be improved
• Tests should be implemented (if possible)

The config currently would look like this:

spring:
  cloud:
    config:
      server:
        git:
          uri: https://github.com/organization/my-config-server-repository
          username: x-access-token # used to authenticate with GitHub App access tokens
          password: |
            -----BEGIN RSA PRIVATE KEY
            ...
            -----END RSA PRIVATE KEY-----
          app: true # indicates that an app should be used
          appId: 530 # the id of the app
          apiUri: https://api.github.com/ # the GitHub api uri (might be changed for enterprise customers)
          installationId: 2164 # the app installation id
          jwtExpirationMinutes: 8 # how long the token is valid until a new one is retrieved

[klopfdreh]

Reference-Discussions:

• https://github.com/orgs/community/discussions/138777
• https://github.com/orgs/community/discussions/24473

[healer0805]

It looks cool