Support more credential types (Credentials Vault?)

Currently, all 3rd party integrations rely on static credentials:

• API key example with Resend
• Token example with GitHub

However, many integrations require standard OAuth 2.0 flows. This limitation makes it challenging to integrate the following examples:

• Client Credentials flow with Auth0: requires exchanging a client_id and client_secret to get an access token.
• Authorization Code flow with HubSpot: requires user interaction (redirect) to get an access token.

Also generic actions like the HTTP Request step are similarly limited, requiring hardcoded credentials.

To solve these limitations, we might need to introduce a Credentials Vault concept to abstract this complexity (handling the token lifecycle and storage) away from individual workflow steps and allow credentials to be reused across integrations.