Add stdin support for convert subcommand to support dockerized workflows

[robberwick]

Problem

The convert subcommand of Trivy doesn't support reading from stdin, which creates significant difficulties when using the dockerized Trivy image in CI/CD pipelines. The lack of stdin support prevents chaining commands in containerized environments.

Impact

When using the Trivy Docker image, the container is ephemeral, making it difficult to work with scan results. For example:

# First scan creates JSON output on the host system
docker run -v /var/run/docker.sock:/var/run/docker.sock -v /var/tmp/trivy_cache:/root/.cache/ aquasec/trivy:0.58.0 image MYIMAGE:MYTAG > result.json

# But then we cannot pipe this into the convert command
# This doesn't work because result.json is on the host, not in the container
docker run aquasec/trivy:0.58.0 convert --format cyclonedx result.json

This forces users to either:

1. Create complex multi-volume mount solutions
2. Use multiple separate commands
3. Create custom scripts to handle file passing

Expected behavior

The convert subcommand should accept input from stdin when:

1. No file argument is provided
2. The argument "-" is explicitly provided (conventional Unix syntax for stdin)

This would enable workflows like:

# Scan and convert in a single pipeline
docker run -v /var/run/docker.sock:/var/run/docker.sock aquasec/trivy:0.58.0 image MYIMAGE:MYTAG -f json | \
docker run -i aquasec/trivy:0.58.0 convert --format cyclonedx -

Target

Container Image

Scanner

Vulnerability