[cache] Layer caching during SBOM generation

[prabhu]

What if cdxgen creates a cache directory to cache the output from the external commands and APIs and reuse it upon subsequent invocation (with the --enable-cache argument or default in CI environments)?

• [ ] Unique key generation could be based on the hashes of the input files used (which are available as a property)
• [ ] Some caching can be performed as a postgen step
• [ ] In CI environments, users could load the cache in advance by utilizing the in-built caching step
• [ ] We can offer commands to clear the cache