[Salesforce]: Add OAuth 2.0 Client Credentials Flow Support

### Integration Name

Salesforce [salesforce]

### Dataset Name

salesforce.setupaudittrail,salesforce.login,salesforce.logout,salesforce.apex

### Integration Version

1.3.0

### Agent Version

8.18.2

### OS Version and Architecture

Rocky Linux 9 (x86_64)

### User Goal

Currently, the Salesforce integration in Elastic Agent supports JWT and User-Password OAuth 2.0 flows for authentication. This request proposes adding support for the OAuth 2.0 Client Credentials flow

Add support for the OAuth 2.0 Client Credentials flow in the Salesforce integration for Elastic Agent, enabling the following:
- Configuration of Salesforce as a connected app with client credentials (client key and secret).
- Option to select the Client Credentials flow as an authentication method.
- Use of the access token obtained via this flow for all Salesforce API interactions.

### Existing Features

Elastic Agent’s Salesforce integration currently supports:
- JWT OAuth 2.0 Flow
- User-Password OAuth 2.0 Flow

It does not support the OAuth 2.0 Client Credentials flow, which is increasingly adopted for automation and backend integrations due to it's simplicity for non-interactive use cases.

### What did you see?

Not relevant for the feature request

### Anything else?

_No response_