[gh-316] Add OAuth flow to MCP servers

[gopar]

Able to sign into MCP servers via OAuth flow, and store keys under ~/.aider/mcp-oauth-tokens.json

Current issue is that refresh tokens are not being taken into account and restart the OAuth flow again.

To test, add an MCP server that requires OAuth such as atlassian (Jira):

{
  "mcpServers": {
    "atlassian": {
      "transport": "http",
      "url": "https://mcp.atlassian.com/v1/mcp"
    }
  }
}

Closes #316