Add Jira Connect public key storage

What does this MR do and why?

This is part of #372967 (closed). See !96818 (closed) for a full context MR.

When a user installs the GitLab for Jira app we receive an installed hook. It includes a JWT token that we have to verify using a public key. The public key is fetched from connect-install-keys.atlassian.com (see lib/atlassian/jira_connect/jwt/asymmetric.rb:15).

To make the app available for self-managed users, GitLab.com will serve as a proxy. It forwards the installed hook to the self-managed instance, but generates a new JWT token. To make this work, we need to:

1. Build the JWT infrastructure (This MR)
   • Add a service to generate JWT tokens.
   • Store the public keys with an expiry date.
   • Provide an endpoint to fetch public keys.
2. Allow the public key CDN URL to be configured. (!98437 (merged))
   • Add an application setting that defaults to https://connect-install-keys.atlassian.com and can be pointed to https://gitlab.com/-/jira_connect/-/jira_connect/public_keys.
3. Forward the installed event to self-managed
   • Add a service that sends an installed hook to the self-managed instance when instance_url is updated.

I explained the problem in more detail in #372967 (closed)

How to set up and validate locally

1. Go to http://localhost:3000/admin/application_settings/network
2. Expand the Outbound requests section
3. Enable Allow requests to the local network from web hooks and services
4. Open a rails console rails c
5. Enable the jira_connect_oauth_self_managed feature: Feature.enable(:jira_connect_oauth_self_managed)
6. Execute the following lines:

# Create a JiraConnect installation
installation = JiraConnectInstallation.create(client_key: '123', shared_secret: '123', base_url: 'https://sample.atlassian.net')

# Generate a new JWT token for the installation
jwt = JiraConnect::CreateAsymmetricJwtService.new(installation).execute

# Fetch the public key ID from the JWT header. The 3rd parameter defines if the decoding should be verified with a public key. In this case, it is not.
key_id = Atlassian::Jwt.decode(jwt, nil, false, algorithm: 'RS256').last['kid']

# Retrieve the public key from storage
public_key_string = Gitlab::HTTP.get('http://127.0.0.1:3000/-/jira_connect/public_keys/' + key_id).body

# Read the public key
public_key = OpenSSL::PKey.read(public_key_string)

# Do a verified decoding of the JWT using the public key
Atlassian::Jwt.decode(jwt, public_key, true, algorithm: 'RS256').first.present?

7. Verify that the last return value is true

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.