Create meta plugin ReadFile with FileFormat dropdown and single-file input

[gravity-api]

Add a new meta plugin named ReadFile that exposes a unified API for extracting text from a single file during an automation run. The user selects a required FileFormat from a streamdown plugins and provides exactly one file source (Path, Url, or Base64). The meta plugin then delegates to the matching streamdown plugin implementation.

This follows the same architecture pattern as SendHttpRequest: users interact only with the meta plugin, while concrete implementations are separate plugins.

Inputs (Meta Plugin)

The meta plugin must accept these parameters:

Name	Type	Mandatory	Description
FileFormat	String	Expression	✅
Path	String	Expression	❌
Url	String	Expression	❌
Base64	String	Expression	❌

Output (Meta Plugin)

The meta plugin must store results in session parameters using consistent names:

Session Parameter	Description
ReadFileText	Extracted text content
ReadFileFormat	Pdf or Docx
ReadFileSource	Path, Url, or Base64

Optional (only if downstream provides it):

• ReadFilePagesCount (Pdf)
• ReadFileParagraphsCount (Docx)

[gravity-api]

Will not be implemented as a meta-plugin for better AI utilization. Each file type will be implemented in an isolated manner.