Keep reading SBOM until encoding is found

[puerco]

What type of PR is this?

/kind bug /kind cleanup

What this PR does / why we need it:

This PR modifies the encoding detection function to read the as much of the SBOM as required to detect the encoding.

Some tools (notably Microsoft's sbom-tool) put the document creation data at the end of the document and we could not detect it by reading just a few bytes from the top.

I pushed another commit removing some debugging output I accidentally checked in.

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

Does this PR introduce a user-facing change?

- bom will now read the SBOM until it detects the SBOM encoding data, enabling it to parse SBOMs with the document data defined at the end of the file.
- When trying to ingest a CycloneDX document, bom will now print a more useful warning 

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cpanato, puerco

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/bom/blob/main/OWNERS)~~ [cpanato,puerco] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment